How to Make It
Preheat the oven to 375°. Spray an 8-inch square baking pan. Line the bottom and 2 sides with parchment paper, allowing the paper to extend 1 inch beyond the pan. Spray the paper.
In a pie plate, toast the hazelnuts in the oven for 12 minutes, until the skins blister. Spread the nuts on a kitchen towel and let cool completely. Rub the nuts together to rub off the skins. Transfer the nuts to a food processor. Add the flour and salt and pulse until the nuts are finely ground.
In a microwave-safe glass bowl, heat the chocolate at 20-second intervals, stirring a few times, until melted. Let cool to warm. In a bowl, beat the butter with the sugar at medium-high speed until fluffy, 2 minutes. Add the chocolate, egg yolks and vanilla and beat until combined. Beat in the dry ingredients just until incorporated.
In another bowl, using clean beaters, whip the egg whites at high speed until soft peaks form. Fold the whites into the batter until no streaks remain. Scrape the batter into the prepared pan and smooth the surface. Bake in the center of the oven for 30 minutes, until a skewer inserted in the center comes out with a few moist crumbs attached. Let the cake cool in the pan for 30 minutes, then freeze until chilled, 1 hour.
Line a baking sheet with parchment paper. Invert the cake onto a cutting board and remove the paper. Cut the cake into 5 equal strips, then cut each strip crosswise into 5 squares, wiping the knife between cuts. Transfer the squares to the prepared baking sheet.
In a microwave, heat the cream and butter until very hot. Whisk in the espresso powder. Add the chocolate and let stand until melted; whisk until smooth.
Using a small offset spatula, spread the ganache on the top and sides of each square. Sprinkle the squares with chopped hazelnuts and refrigerate until the ganache is firm, about 30 minutes.