Reply to "Using Cold vs. Room Temperature Butter in Cookie Dough"

It's small, yet visible chunks of butter in pastry that create flakiness - i.e., as CT Cookie said, when those chunks melt in the heat of the oven, they release steam that causes the dough layers to separate and become flaky. Generally, flaky is not what one wants in cakes or cookies, so the butter is creamed (beaten) to break up any small butter particles. By beating/creaming, a fluffy emulsion of fat-liquid gets created that results in a finer, yet still aerated crumb. You cannot create such an emulsion with melted butter - the same fluffy structure will not result - which is why melted butter is generally (though not always) avoided in most cake recipes.