In a medium bowl sift or whisk the flour, cocoa powder and salt together. Set aside.
In a large bowl, using a mixer, beat the butter and sugar for 2-3 minutes until well combined. add the egg yolks and keep beating until combined. Add the flour mixture and beat slowly until the ingredients come together into a ball of dough.
Turn dough to a lightly floured surface and form into a ball. Divide ball to half, then flatten each one slightly with your hands to form a disc (the shape doesn’t matter). Wrap each disc with plastic wrap and refrigerate for at least 1 hour until firm.
Preheat the oven to 340°F/170°C. Line baking sheets with parchment paper and set aside.
Roll out each dough on a floured surface until it’s 1/8 to 1/4 inch thick (for sandwich cookies I prefer thin cookies). Cut rounds of dough using a 2 inch (5 cm) cookie cutter, and gently transfer them to the prepared baking sheet, placing them 1 inch apart. If the dough becomes soft, place in the fridge or freezer for several minutes before baking.
Pipe a layer of ganache onto the flat side of half the cookies (you can use a pastry bag or a teaspoon). Place remaining cookies on top and press gently.
Store cookies at room temperature in an airtight container for up to 2 days (no longer because of the dairy in the ganache) or in the refrigerator for up to a week.