That would be fine, although, I've tried it, and I prefer the milk, since it's neutral,
and I like the contrast of the vanilla sponge cake to the deep chocolate. (The milk idea was given to me awhile back from a pastry chef). But, the coffee will be good, too, plus coffee is a tenderizer - I think some reviewers on Epicurious used it, and some others used Kahlua.
Also, as I mentioned awhile ago, some reviewers replaced all the butter with cream cheese, but I thought it would taste too much like a chocolate cheesecake, so for the past few times I replaced just 2 Tbsp of the butter with cream cheese, and it didn't interfere with the original chocolate taste.