Chocolate-Hazlenut Thumbprints

4.5 oz all-purpose flour (about 1 cup)
1 c powdered sugar
1/3 c unsweetened cocoa
1/4 t salt
1/2 c butter, softened
2 large egg yolks
1/2 t vanilla extract
2/3 c finely chopped hazelnuts, toasted
1/3 c Nutella

Preheat oven to 350F. Weigh or lightly spoon flour into a dry 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. Add the yolks 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, pressing 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 350F for 10 minutes. Remove cookies from pans; cool completely on wire racks. Smooth a scant 1/2 teaspoon of Nutella into center of each cookie. Yield: 28 cookies (serving size: 1 cookie).