Sift together flour, cocoa, baking soda, and salt in a bowl.
Beat Nutella and butter with an electric mixer until smooth and combined. Beat in the brown sugar, then egg and hazelnut extract (if using). Add the flour mixture with mixer on low speed until dough comes together.
Transfer dough to a large sheet of plastic wrap or wax paper. Use the wrap to shape into a 2-inch-diameter log. Secure tightly, and chill for 2 hours, or overnight.
Preheat oven to 350°F 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 9-11 minutes, or until cookies are firm and dry. Transfer to wire rack to cool.
Decorate with 1/2 tsp Nutella and 1 hazelnut per cookie.