Preheat the oven to 350

Spoon flour into a measuring cup and level the top with a knife to create exactly 1 cup of unpacked flour. In a large bowl, combine flour with powdered sugar, salt and cocoa, stirring until mixed. In a separate large bowl, place the stick of butter and beat it with a hand mixer at a medium speed until fluffy (about 1-2 minutes). Stir the egg yolks together in a small cup with a whisk and when combined, add them along with the vanilla to the butter and beat with mixer until combines. Add flour mixture to bowl and beat together on a low speed for several minutes until dough combines and thickens.

Place the dough onto a sheet of wax paper and knead it several times until smooth. Breaking pieces off the dough, form 28 balls (about 1 inch each). Line a large baking sheet (or 2 if necessary) with parchment paper and place the balls in rows one inch apart on the paper. In the center of each cookie, press your thumb about halfway through the ball to create an indentation. Bake cookies for 10 minutes and remove from oven. Place cookies on wire racks and let them cool completely. Once cooled, spread about ½ teaspoon of Nutella into the indented center of each cookie.