How to Make It
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.
Divide the dough into quarters. On a lightly floured surface, working with 1 piece at a time and keeping the rest refrigerated, roll out the dough 1/8 inch thick. Using a 2 3/4-inch round cookie cutter, stamp out as many cookies as possible and transfer to the baking sheets. Decorate half of the cookies with peppermint candies or decorate all of the cookies with colored sugar or sprinkles. Repeat with the remaining dough and decorations.
Bake 1 sheet of the cookies in the upper and 1 sheet in the lower third of the oven for 14 minutes, or until firm; shift the pans from top to bottom and front to back halfway through. Slide the parchment onto wire racks and let the cookies cool completely. Repeat with the remaining cookies.
In a medium bowl, beat the remaining 4 tablespoons of butter with the cream cheese until smooth. Beat in the confectioners' sugar, mint extract and the remaining 1/2 teaspoon of vanilla extract. If you used the peppermint candies, spread 1 1/2 teaspoons of the filling on the undecorated cookies and top with the peppermint candy-decorated cookies. If you used the colored sugar or sprinkles, spread 1 1/2 teaspoons of the filling on the undecorated side of the cookies; top the filling with the remaining cookies.