From 6 Bittersweets
Adapted from Aromas Y Sabores
Makes about 3 dozen 1 1/2-inch cookie sandwiches
1 cup all purpose flour
3/4 cup cornstarch
1/4 cup unsweetened cocoa powder
1/8 tsp salt
1 tsp baking powder
7 Tbsp unsalted butter, softened
2/3 cup sugar
1 large egg, room temperature
1 large egg yolk, room temperature
1 tsp pure vanilla extract
1 tsp brandy (optional)
1/2 cup Nutella or other chocolate hazelnut spread
1/2 to 3/4 cup hazelnuts, toasted and finely chopped
Sift together flour, cornstarch, cocoa powder, salt, and baking powder. Set aside.
Using a handheld electric or stand mixer, beat butter and sugar together until light and fluffy. Add whole egg and yolk and beat until incorporated. Add vanilla and brandy and mix well. Finally add the flour mixture and mix by hand using a wooden spoon or spatula until you get a soft dough. Refrigerate for at least 2 hours, preferably overnight.
Preheat the oven to 275 degrees F. Divide dough into 3 equal parts, and work on one at a time while keeping the rest covered. On a floured work surface, roll out dough to desired thickness, between 1/16 and 1/8-inch. Cut into rounds (traditional shape) or whatever shape you choose. Save scraps to re-roll (only once) and use at the end. Transfer to a baking sheet lined with parchment paper or a silicon mat. Bake for 12 to 16 minutes, depending on size of cutter. Cookies should be just cooked but not browned. Repeat rolling, cutting, and baking cookies until all the dough is used up. Let all cookies cool completely.
Transfer chopped hazelnuts into a medium bowl; set aside. Once cookies have cooled, spread a spoonful of Nutella on one cookie and top it with another of similar shape and size. Press together gently, then roll edges of cookie sandwich in chopped hazelnuts. Repeat with remaining cookies.