Since “like dissolves like”, the fat in milk is able to dissolve the fat in the cocoa powder. Higher-quality cocoa powder has a higher proportion of cocoa butter, so more of it is readily soluble in the milk fat. With lower-quality cocoa powder, more of it is made of starch. As you might have seen with, say, throwing corn starch into a sauce, starch tends to clump in contact with moisture.
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