Preheat oven to 350* F.

Lightly spoon flour into measuring cup; level with a knife. Combine flour, sugar, cocoa, and salt; stir with a whisk. Place butter in a large bowl, and beat with a mixer at medium speed until light and fluffy (about 2 minutes). Stir egg yolks with a whisk, adding, espresso if desired. Add the yolk mixture and vanilla to butter; beat well. Add flour mixture to butter mixture; beat at low speed just until combined.

Turn dough out onto a sheet of wax paper, knead 6 times or until smooth and shiny. Shape dough into 28 (1 inch) balls. Roll sides of balls in nuts, bressing gently. Arrange balls 1 inch apart on baking sheets lined with parchment paper. Press thumb into center of each cookie. leaving an indentation. Bake, 1 batch at a time, at 350* F, for 10 minutes. Remove cookies from pans, cool completely on wire racks. Spoon a scant 1/2 tsp Nutella into center of each cookie.