Gemara

(Mishnah): Any amount if the following forbid a mixture - ... Basar v'Chalav (meat cooked with milk...)

Chulin 108a (Rav): Efshar Lesochto (if food 'A' received taste from an Isur and became forbidden, and then 'A' was cooked with enough Heter for Bitul,) it is forbidden (we do not say that the forbidden taste exudes).

(Shmuel): It is permitted.

Contradiction (Rav): If a K'Zayis of meat fell into a cauldron of milk, the meat is forbidden, and the milk is permitted.

If Rav forbids Efshar Lesochto, since some milk was absorbed in the meat and became forbidden, when it exudes from the meat, it should forbid all the milk!

Answer #1:It says "do not cook a kid in its mother's milk." If one did so, only the kid becomes forbidden, but not the milk.

Rejection: Rav taught that if one cooked half a k'Zayis of meat with half a k'Zayis of milk, one who eats (all of) it is lashed.

If only the meat becomes forbidden, he cannot be lashed for eating half a k'Zayis of Isur!

Answer #2: Rather, Rav holds that also the milk becomes forbidden. The case is, the meat fell into a pot of boiling milk (and was removed while the milk was still boiling). The meat only absorbs. It does not emit (milk that it absorbed).

Rishonim

Rambam (Hilchos Ma'achalos Asuros 16:6): A piece of Basar v'Chalav, or Chulin b'Azarah, which is Asur b'Hana'ah mid'Rabanan, forbids any amount (of a mixture) until it is removed.

Tosfos (Chulin 100a DH beshe'Kadam): When a piece received taste from Nevelah, it is as if the Isur is intact, and it forbids all pieces b'Mino. If the Nevelah did not give taste to the piece, it is as if the Isur is not intact. It suffices to be Mevatel the Isur; it forbids only the first piece. Only when milk falls on a piece, it becomes like Nevelah, and we need 60 times as much as the piece. This is because each (meat and milk) is permitted by itself, and together they are forbidden. Therefore, the meat itself becomes Isur. One is lashed for half a k'Zayis of milk cooked with half a k'Zayis of meat.

Tosfos (108b DH Amai): The Gemara suggested that milk absorbed in the meat should forbid all the milk when it exudes. The Rashbam inferred that according to the opinion that Min b'Mino forbids any amount, if Nevelah was cooked, we need 60 times as much as gravy as Nevelah to be Mevatel the taste of Nevelah that exudes, and 60 times as much as in the piece of Heter to be Mevatel the gravy that exudes from the Nevelah. The Ri disagreed, for the Gemara answered that the Torah forbids only the meat, but not the milk. Mid'Rabanan, also the milk becomes Nevelah, like for all Isurim! Rather, since it is forbidden only due to the taste of meat absorbed, it is not like Tamei milk. Rather, it is Batel in the milk, just like taste of the forbidden meat is Batel in the milk. It is not considered Min b'Mino. Likewise, juice that exudes is Batel in the gravy, just like the taste of the Nevelah. It is not considered Min b'Mino.

Poskim

Shulchan Aruch (YD 87:11): If cheese was curdled in the skin of a Kosher stomach, it is forbidden only if it has the taste of meat. If it was curdled in the skin of stomachs of a Nevelah, Terefah or Tamei animal, it forbids any amount (of a mixture).

Rema: This is because something intrinsically forbidden that curdles is not Batel even in 1000. This is if the only Ma'amid (curdling agent) was Asur. If there was also a permitted Ma'amid, this is Zeh v'Zeh Gorem, so it is permitted if there is 60 times as much Heter.

Shulchan Aruch (101:2): The law of Chatichah ha'Re'uyah Lehiskaved applies only to something forbidden due to itself, such as Nevelah or Basar v'Chalav.

Beis Yosef (DH u'Mah): Even though the Gemara connotes that a piece of Basar v'Chalav is not Batel when it is Re'uyah Lehiskaved, there is different. Since they were cooked together, they become like one mass of Isur. This is unlike other Isurim.

Bach (3): Even R. Tam and the Ri, who hold that Chatichah Atzmah Na'asis Nevelah applies to all Isurim, and not only to Basar v'Chalav (unlike R. Efrayim), agree that they do not resemble Nevelah and do not forbid due to Chatichah ha'Re'uyah Lehiskaved. They can become Batel. Tosfos (108b DH Amai) explicitly says so.

Bach (3 DH Kasav): This is only if the Isur is due to itself, e.g. fowl cooked with milk. If there is a Safek whether it is Chatichah ha'Re'uyah Lehiskaved, even if the Isur is mid'Oraisa, it is Batel. The stringency of Chatichah ha'Re'uyah Lehiskaved is only mid'Rabanan, therefore we are lenient about a Safek.

Taz (3): This is because absorbed taste is not Re'uyah Lehiskaved.

Taz (4): The Rema (Toras Chatas 43) says that cheese of Nochrim if forbidden lest Tamei milk was mixed in. (It does not harden; it remains in the holes of the cheese.) It (the Tamei milk) is not Re'uyah Lehiskaved. We are not concerned lest the milk was from Trefos, because most animals are Kosher. However, in a place where there is concern lest they curdle with the skin of stomachs of Nevelos, it is Chatichah ha'Re'uyah Lehiskaved, since Basar v'Chalav applies, and it is not Batel.

R. Akiva Eiger: Above (87:11), we permit cheese curdled in the skin of a Kosher stomach if there is 60 times as much milk. This shows that curdling does not make Basar v'Chalav. If so, curdling in the skin of the stomachs of a Nevelah is forbidden only due to Nevelah, so it is not an intrinsic Isur. Also, the Taz (87:2) said that Chatichah Atzmah Na'asis Nevelah does not apply to meat of a Tamei animal with milk, since we cannot say that each is permitted by itself. Therefore, the milk is not forbidden due to itself, rather, due to the absorbed taste of meat. Therefore, it is not Chatichah ha'Re'uyah Lehiskaved. If so, curdling in the stomach of a Nevelah is more lenient, for Basar v'Chalav does not make it intrinsically forbidden, since one of them was already forbidden mid'Oraisa. This requires investigation. Regarding the first question, we can say that we are concerned lest there was not 60 times as much milk. The second question is difficult.

Minchas Yakov (on Torah Chatas, 40:10): Basar v'Chalav is called an intrinsic Isur only if they were cooked together, but not through salting or pickling, like it says in Isur v'Heter 46:4. (However, Isur v'Heter 25:5 says that it is called an intrinsic Isur through pickling, like it says in Isur v'Heter 46:4.) The Rashba, Ran, and all the Poskim brought in the Beis Yosef in this Siman connote like this. Hagahos Sha'arei Dura says that we discuss (pouring from a Kli Rishon), which cooks Kedei Kelipah (the thickness of a layer that could be peeled off). This is proper Isur, for it is properly cooked.

Rebuttal (R. Akiva Eiger): He holds that if meat and milk were pickled together, it is not Chatichah ha'Re'uyah Lehiskaved. I hold that his proof is invalid. It is clear from the Taz (4) that Chatichah ha'Re'uyah Lehiskaved applies to Basar v'Chalav mid'Rabanan.

Noda bi'Yehudah (1 YD 30, cited in Pischei Teshuvah 3): A case occurred in which Rachel lent to Leah kneading bowls to bake in them. Leah did not know that they were used for meat. She baked in them dumplings with butter and returned them to Rachel. Rachel baked dumplings with lard in nine bowls, including the two that Leah baked in. They were clean. Seemingly, we cannot say that the dumplings baked in the borrowed bowls are Batel in the majority, for they are Re'uyah Lehiskaved, and anything normally counted is not Batel. Even though something forbidden due to absorptions is not called Chatichah ha'Re'uyah Lehiskaved, Basar v'Chalav is Re'uyah Lehiskaved. However, this is a big loss, so I investigated further.

Noda bi'Yehudah: Perhaps Basar v'Chalav is only when milk is absorbed in a piece of meat. Here, the primary matter is the dough. The butter and lard are forbidden, and they are called intrinsic Isur, but they are merely Belu'ah (enveloped) in the dough. Regarding something Belu'ah, we do not apply Chatichah ha'Re'uyah Lehiskaved.

Noda bi'Yehudah: If only Kedei Kelipah or Kedei Netilah (the thickness of a finger) is forbidden, we do not apply Chatichah ha'Re'uyah Lehiskaved or Davar sheb'Minyan. Here, even if the two dumplings were Vadai baked in the borrowed bowls, perhaps we could permit them through peeling off a layer. The Shach (105:23) says that the Darchei Moshe says that a Kli that absorbed Isur forbids what it touches only Kedei Kelipah if there is no gravy. The Shach concluded that if a Tahor Kli absorbed forbidden oil, perhaps even if it is totally dry it forbids up to 60. He was unsure, but the Magen Avraham (451:37) was sure that it does not, so we rely on the Magen Avraham. If so, the dumplings are Batel in the majority. I am not so certain about this, because the dumplings were smeared with lard before they were baked. The lard became forbidden, and forbids the dumplings. However, presumably the dumplings are 60 times the amount of intact lard. There is no concern for an Isur Torah here, since the lard was of fowl.

Noda bi'Yehudah - Conclusion: I am sure about the first reason, therefore I perhaps through Bitul b'Rov. Also, the butter is b'Eino Mino with respect to the lard, so even for a dry mixture it is Batel in 60. Since this is an Isur mid'Rabanan, it is Batel in a simple majority.