[twic]

Depends on the shape of the cup! You can contrive a cup shaped like an exponentially flaring horn, where adding the milk increases the volume a little, which massively increases the surface area, and so leads to faster cooling. Or you can have a cup with a converging top, like a brandy glass, where adding the milk reduces the surface area, and makes cooling even slower.

[j_bum]

And add the twist: what if the cup is made from a highly conductive material?

[IshKebab]

Clever, but it's implicitly a normal cylindrical cup.