This is Sam's favorite cookie, hands down. But beware; they're very rich.
1 stick unsalted butter (8 tbsp), room temperature
1/2 cup granulated sugar
1 large egg, room temperature
1 cup unsweetened cocoa powder
1/2 cup plus 2 Tbsp. all-purpose flour
Confectioner's sugar, for rolling out dough
Cream butter & sugar on medium speed until completely smooth (~3 minutes). Add egg & mix until well combined.
In separate bowl, whisk together cocoa powder and flour. Add flour mixture to butter mixture; mix on low speed until combined, scraping down the sides of the bowl once. Chill the dough until slightly firm. Divide the dough into 3 balls. Roll out each between two layers of wax or parchment paper to 1/8-inch thickness. Chill. Use a 2-inch round cutter to cut out cookies. Space 1/2 inch apart for baking on parchment or silicone-lined baking sheet. Re-roll trimmings and chill. Repeat as long as you have dough.
Bake the cookies until you smell chocolate, about 12 minutes. Cool on wire rack. Fill with Nutella from the jar.