- 1 1/3 cups blanched hazelnuts
- 1 1/2 cups plus 1/3 cup sugar
- 1 tablespoon kosher potato starch
- 6 large egg whites
- 1/4 teaspoon salt
- 1/4 cup whole milk
- 5 cups heavy cream
- 8 ounces bittersweet chocolate, coarsely chopped, plus shaved chocolate for topping
- 1 tablespoon instant espresso powder
- Position racks in the upper and lower thirds of the oven and preheat to 325 degrees F.
- Spread the hazelnuts on a baking sheet and bake until toasted, 12 to 15 minutes; let cool completely.
- Combine 1 cup each hazelnuts and sugar and the potato starch in a blender or food processor and pulse until very finely ground.
- Transfer to a large bowl.
- Reduce the oven temperature to 275 degrees F.
- Line 2 large baking sheets with parchment paper.
- Using a bowl as a guide, draw three 7-inch circles on the parchment with a pencil (2 circles on 1 piece and 1 circle on the other); flip the parchment so the markings face down.
- Put the egg whites, 1/2 cup sugar and the salt in a large heatproof bowl.
- Set over a saucepan of simmering water (do not let the bowl touch the water) and whisk until the sugar is dissolved and the mixture is warm, 2 to 5 minutes.
- Remove the bowl from the saucepan and beat with a mixer on medium-high speed until stiff and shiny but not dry, about 2 minutes.
- Fold the milk and one-quarter of the egg white mixture into the nut mixture with a rubber spatula until smooth, then fold in the remaining egg whites until just combined.
- Divide the batter among the 3 circles on the parchment and spread evenly with an offset spatula to fill the circles.
- Bake 1 hour, 30 minutes, then switch the position of the pans and continue baking until firm, dry and golden, 1 to 2 more hours. (The layers are done when they easily peel off the parchment.)
- Let cool slightly on the baking sheets; peel off the parchment and transfer to a rack to cool completely.
- Meanwhile, make the ganache: Put 1 cup cream in a microwave-safe bowl; microwave until very hot, about 2 minutes.
- Whisk in the chocolate and espresso powder until smooth.
- Let cool to room temperature, then cover and refrigerate until cold, thick and spreadable, at least 30 minutes.
- Make the whipped cream: Combine the remaining 4 cups cream and 1/3 cup sugar in a large bowl and beat with a mixer on medium speed until it barely holds soft peaks, 1 to 2 minutes.
- Finish beating the cream by hand with a whisk until soft peaks form; do not overbeat.
- Put 2 meringue layers on a baking sheet; spread half of the ganache on each, to the edge.
- Refrigerate until set, 15 minutes. (If the ganache is too firm to spread, microwave in 20-second intervals to soften.)
- Place one of the ganache-topped meringues on a platter.
- Spread with 1 cup whipped cream.
- Stack the other ganache-topped meringue on top; spread with another layer of whipped cream.
- Top with the last meringue, then cover the top and side of the cake with the remaining whipped cream.
- Chop the remaining 1/3 cup hazelnuts and sprinkle on top of the cake along with the shaved chocolate.
- Refrigerate at least 6 hours or overnight before serving.