In this recipe, I used Dutch-process cocoa powder for its dark color and smooth finish. To me, natural cocoa powder has a somewhat fruity chocolate flavor. Generally speaking, the two are not interchangeable. If you're in a pinch, and the recipe calls for less than 3/4 cup, you can substitute natural cocoa powder for Dutched, but not the other way around.
For the cookies: Preheat the oven to 350 degrees F. Line 2 baking sheets with parchment paper.
In a large bowl, whisk together the flour, cocoa, salt and baking powder. In another large bowl using an electric mixer, beat the butter and granulated sugar on medium speed until fluffy, about 3 minutes. Add the yolks and beat until combined. Add the flour mixture to the butter mixture and beat until well combined. Divide the dough into 2 pieces.
Lightly dust a piece of parchment with cocoa powder. Roll 1 piece of dough on the prepared parchment to between 1/4 inch and 1/8 inch thick. Using a 2 1/4-inch round, fluted cookie cutter, stamp out as many cookies as you can. Transfer the cookies to the prepared sheets. Roll and cut the remaining dough; you can reroll the scraps once. Chill the unbaked cookies for 10 to 15 minutes.
Bake until the cookies are set and the tops look dry, 14 to 16 minutes, rotating the sheets halfway through cooking. Transfer the cookies to a rack to cool completely, about 45 minutes.
For the filling: Beat the confectioners' sugar and butter with an electric mixer until smooth. Add just enough milk to make a creamy filling. Beat in the mint extract.
When the cookies are cool, flip half of them over. Spread or pipe about 1 heaping tablespoon of the filling on the flipped cookies. Top with the other halves and press together gently.