These are wonderfully simple cookies that act as a blank canvas for all kinds of flavors—grate some lemon or orange zest into the butter-sugar mixture if you like, or swap a different flavored extract for the vanilla. To make chocolate cookies, swap 1/4 cup cocoa for 1/4 cup of the flour. To decorate, roll the balls of dough in sprinkles or coarse sugar before baking. To make birthday cake cookies, stir an additional 1/4 cup of sprinkles into the dough itself.