Preheat the oven to 375°. Line 6 baking sheets with parchment paper. In a medium bowl, sift the flour with the cocoa, baking powder and salt. In a large bowl, using a handheld electric mixer, beat 2 sticks of the butter until creamy. Add the sugar and beat until light and fluffy. Add the egg, egg yolk and 1 teaspoon of the vanilla extract and beat until blended. Add the dry ingredients and beat at low speed until moistened and a stiff, crumbly dough forms. Scrape the dough onto a sheet of plastic wrap, pat into a disk, wrap and refrigerate until chilled.