4.5 ounces all-purpose flour (about 1 cup) $
1 cup powdered sugar
1/3 cup unsweetened cocoa
1/4 teaspoon salt
1/2 cup butter, softened $
2 large egg yolks $
1 teaspoon instant espresso (optional)
1/2 teaspoon vanilla extract $
2/3 cup finely chopped hazelnuts, toasted
1/3 cup hazelnut-chocolate spread (such as Nutella) $
1. Preheat oven to 350°.
2. 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, 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.
3. 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 350° for 10 minutes. Remove cookies from pans; cool completely on wire racks. Spoon a scant 1/2 teaspoon hazelnut-chocolate spread into center of each cookie.