The heat is all contained within the box. It's matters of the natural world that help get whatever is within to heat up. In the case of butter, the shape will also change, but if I say, put milk in it, it would only get warmer.

[Alucard can't help but watch now. Sypha's face is something he's never seen in reaction to something his father has done (save for himself or his mother.) It's delighted, fascinated. Curious.]

The technology will be figured out by everyone else at some point, I'm sure.