In a bowl of a standing mixer fitted with a paddle attachment, beat together the butter, sugar, vanilla extract, and egg yolks. Mix until smooth and creamy.
In a small bowl, sieve together the flour, cocoa powder, and salt. Turn the mixer to the low speed, and add the flour mixture gradually, spoon by spoon. The batter will be very thick.
Place the dough between two sheets of baking paper, and using a rolling pin spread the dough until it reaches around 0.5cm thickness.
Place it flat in the fridge and chill the dough for about 15 minutes.
In the meantime, line the baking sheets with the silicon mats, and preheat the oven to 170°C (340°F).
Use the cookie cutter to cut out the cookies and place them on the silicon mats leaving some space in-between.
Place the cookies in the oven and bake for 8-10 minutes. They will be slightly soft, but they will firm up as they cool down.
