- 1 cup hazelnuts, toasted and skinned
- 2 1/2 cups all-purpose flour
- 1/2 teaspoon salt
- 1/2 teaspoon baking powder
- 1 cup unsalted butter, softened
- 1 1/2 cups granulated sugar
- 1 large egg plus 1 large egg yolk
- 4 teaspoons instant espresso powder
- 2 teaspoons vanilla extract
- 3/4 cup heavy cream
- 3 cups bittersweet chocolate chips
Process hazelnuts in a food processor until finely ground, about 30 seconds. Whisk flour, salt, baking powder, and ground hazelnuts together in a bowl.
Using a stand mixer fitted with a paddle attachment, beat butter and sugar on medium-high speed until pale and fluffy, about 3 minutes. Add egg and yolk, one at a time, espresso powder, and vanilla and beat until combined.
Reduce speed to low and add flour mixture in 3 additions until just combined, scraping down bowl as needed. Transfer dough to counter and divide in half. Form each half into a 5-inch disk, warp disks tightly in plastic wrap, and refrigerate for 1 hour.
Preheat oven to 375°F. Line 2 baking sheets with parchment paper. Let chilled dough soften on counter, about 10 minutes. Roll 1 disk of dough into a 14-inch circle, about 1/8 inch thick, on a lightly floured counter. Using a 2 1/2-inch round cookie cutter, cut out 30 circles; space circles 1/2 inch apart on prepared sheets. Gently reroll scraps once, cut into circles, and transfer to prepared sheets.
Bake until edges are slightly browned, about 7 minutes, rotating sheets halfway through baking. Let cookies cool on sheets for 5 minutes, then transfer to wire rack. Repeat with second disk of dough. Let cookies cool completely.
Heat cream in a small saucepan over medium heat until simmering. Place 1 3/4 cups chocolate chips in bowl. Pour hot cream over chocolate chips; cover and let sit for 5 minutes. Whisk chocolate mixture until smooth. Refrigerate ganache, uncovered, stirring occasionally, until thickened, about 40 minutes.
Spread 2 teaspoons of ganache over bottom half of cookies, then top with remaining cookies, pressing lightly to adhere. Microwave remaining 1 1/4 cups chocolate chips in a bowl at 50% powder, stirring occasionally, until melted, 2 to 3 minutes. Drizzle chocolate over cookies and let set, about 30 minutes, before serving.