Date: 25 Av
From: Rabbi Moshe Sofer
To: Rabbi Elazar
The Chasam Sofer rules that chelev cooked in milk is forbidden in benefit, following the Kanfei Yonah and majority of Acharonim against the Rambam's lenient view. Tallow candles mixed with butter are nullified in a mixture, but actively lighting them is problematic as it may constitute bishul basar bechalav — an act the Torah prohibits in itself, even though such flame-based cooking does not qualify as bishul for Shabbos melacha purposes.
Source: Shu"t Chasam Sofer YD §92