Nutella, the creamy chocolate-hazelnut spread, does double duty as dough ingredient and frosting here.
1. Sift together flour, cocoa, baking soda, and salt in bowl.
2. Beat Nutella and butter with electric mixer until smooth and combined. Beat in brown sugar, then egg and hazelnut extract, if using. Beat in flour mixture with mixer on low speed until dough forms.
3. Transfer dough to large sheet of plastic wrap or wax paper. Shape dough into 2-inch-diameter log with plastic wrap. Wrap tightly, and chill 2 hours, or overnight.
4. Preheat oven to 350ºF. Slice dough log into 1/4-inch-thick slices, rolling log 90 degrees between slices to keep edges round. Transfer slices to greased or parchment paper–lined baking sheet. Bake 9 to 11 minutes, or until cookies are firm and dry. Transfer to wire rack to cool.
5. Frost each cookie with 1/2 tsp. Nutella, and garnish with 1 hazelnut.