Hazelnut-Crusted Chocolate Sandwiches

Cookies: In a bowl, whisk together flour, cocoa, sugar, baking powder, and salt. In a large bowl, whisk together butter and egg. Add brown sugar, and whisk to combine. Gradually add to flour mixture and stir with a spatula until liquid is absorbed. Transfer to a sheet of plastic wrap.

Divide dough into thirds; roll out each portion 1/16-inch thick on a nonstick baking mat, topped with plastic wrap. Stack on a baking sheet, and refrigerate until firm, about 30 minutes.

Preheat oven to 350 degrees. Using a 2-inch cookie cutter, cut dough into rounds; transfer to parchment-lined baking sheets. Reroll scraps once and cut more rounds, to make a total of 72 cookies. (If dough becomes soft, freeze until firm.) Bake cookies, rotating sheets halfway through, until firm and fragrant, 9 to 10 minutes. Transfer sheets to wire racks and let cool completely. 

Filling: In a large bowl, beat butter, confectioners’ sugar, vanilla, and salt, starting on low speed, then increasing to high for 2 minutes, scraping down sides of bowl occasionally. Stir in melted chocolate to combine.

Spread 2 tablespoons filling each on half of cookies, then sandwich with remaining cookies. Roll edges in chopped hazelnuts to adhere. Chill overnight before serving. (Assembled cookies can be refrigerated in an airtight container up to 2 days.)