Melt 4 tablespoons of butter in medium saucepan over medium heat.
Add cocoa powder and espresso powder; stir until mixture forms smooth paste. Set aside to cool 15 - 20 minutes.
In bowl of stand mixer fitted with paddle attachment, mix remaining 16 tablespoons butter, sugar, salt and cooled cocoa mixture on high speed until well combined and fluffy, about 1 minute.
Add yolks and vanilla and mix on medium speed until combined.
With mixer running on low, add flour in 3 additions, waiting until each addition is incorporated before adding next. Continue to mix until dough forms cohesive ball.
Turn dough onto work surface; divide into three 4-inch disks. Wrap each disk in plastic wrap and refrigerate until dough is firm, yet malleable, 60 minutes.
Line large baking sheet with parchment paper.
Adjust oven rack to middle position and heat oven to 375°
Roll out 1 dough disk between 2 large sheets of parchment paper to even thickness of 3/16 inch. Peel parchment from one side of dough and cut into desired shapes using cookie cutters.
Place on prepared baking sheet, spacing them about 1 inch apart.
Bake until cookies offer slight resistance to touch, 10 - 12 minutes.
Cool for 5 minutes, then using spatula, transfer cookies to wire rack and cool completely.