Heat oven to 350°F degrees. Line two baking sheets with parchment paper.

Sift the flour and cocoa together and set aside. In the bowl of a standing mixer with a paddle attachment, beat the butter, sugar, and salt together until light and fluffy. Add the yolks and vanilla and mix until combined, scrape down sides and mix briefly again. Add the flour mixture a little at a time then mix until combined.

Transfer the dough to a lightly floured work surface and divide into two equal pieces. If the dough is too soft to handle, wrap and chill it until firm enough to roll out (No more than 30 minutes or the dough will become crumbly). Roll each batch a 1/4-inch thick and cut into desired shapes and transfer them to the prepared baking sheet. Use a wooden skewer (or other pointy object like the tip of a thermometer) to poke the cookies with holes. For my round cookies I did 14 holes. Sprinkle with sugar if desired.

Bake the cookies for 16 to 18 minutes, or until they stay firm when tapped in the center. Transfer to a wire rack to cool completely. Repeat with remaining cookie dough, rerolling scraps as needed.