Hazelnut Chocolate Cloud Cake
1 cup blanched hazelnuts (5 oz.)
4 oz. unsweetened chocolate (about 3/4 cup chopped), melted and cooled
4 large egg yolks
3/4 cup sugar, plus additional for rolling
1/4 cup cornstarch
1/2 vanilla bean, split and seeds scraped
2 cups whole milk, room temperature
14 oz. puff pastry (preferably all-butter, such as Dufour), thawed
1/2 cup cream
Hazelnut praline, for serving
Caramel-dipped hazelnuts, for serving
Vanilla ice cream, for serving
Cotton candy, for serving (optional)
Preheat oven to 375 degrees. Roast hazelnuts until golden brown, 10 to 12 minutes. Cool slightly.
Transfer hazelnuts to bowl of food processor and pulse until finely ground, about 30 seconds. Add chocolate and process until smooth, about 1 minute. Transfer to large heatproof bowl; set aside.
Add yolks, sugar, cornstarch, a pinch of salt, and vanilla bean and seeds to a small saucepan and whisk to combine. Add milk and whisk until smooth. Bring to a boil over medium heat, whisking constantly (about 10 minutes). Continue cooking until mixture is thick and glossy, 1-2 minutes more. Strain through a fine mesh sieve over the chocolate mixture and stir to combine. Press a piece of plastic wrap directly on the surface and refrigerate until cool, at least 2 hours and up to overnight.
Meanwhile, make the puff pastry discs: Place the puff pastry on a well-sugared surface, sprinkle top generously with more sugar. Roll out to 1/8" thick (about a 10"x 16" rectangle). Cut eight 4" circles using a biscuit cutter and transfer to a parchment-lined baking sheet. Freeze 15 minutes. Place a second sheet of parchment paper on top of the puff pastry and a second baking sheet on top of the parchment. Bake 30 minutes until set and beginning to brown, then carefully remove top baking sheet and parchment paper. Bake until deep golden brown, about 5 minutes longer. Transfer to a wire rack to cool completely. Puff pastry discs can be baked ahead and kept in an airtight container up to one day.
Stir hazelnut custard with rubber spatula to loosen. Whip cream to soft peaks and fold into custard in three additions. Top each puff pastry disc with about 1/2 cup of hazelnut custard, smoothing top and sides with an offset spatula. Once assembled, tarts can be refrigerated in an airtight container for up to one day. Just before serving, garnish with hazelnut praline and caramel dipped hazelnut. Serve with vanilla ice cream and cotton candy.