Firstly, milk masks and dims the flavour of cocoa. “With milk, there’s a little bit of sweetness, a little bit of sourness, so there are some other contributing flavors present there. Which, in the case of chocolate cake, kind of works against it,” explains King Arthur’s senior recipe Developer Molly Marzalek-Kelly.
Article source: https://www.huffingtonpost.co.uk/entry/boiling-water-moist-chocolate-cake_uk_6aedbacbe4b043b6d4e158c5