Special equipment: a candy thermometer; 2 pastry bags; small round silicone molds
For the hazelnut sugar crust: Preheat the oven to 320 degrees F and line a baking sheet with parchment paper or a silicone baking mat.
In the bowl of a stand mixer fitted with the paddle attachment, mix together the butter, all-purpose and hazelnut flours, turbinado sugar, baking powder, salt and egg until just combined. Roll thin between 2 pieces of parchment paper or on a silicone mat. Cut into 1-inch rounds and transfer to the prepared baking sheet. Bake for 10 minutes, then transfer to a rack to cool.
For the sacher sponge cake: Preheat the oven to 335 degrees F and line a large rimmed baking sheet with parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment, mix the almond paste and confectioners' sugar for about 1 minute. Slowly add the egg yolks until well combined. Add the cocoa powder and flour and mix until combined. Add the melted butter and mix until combined.
In a clean mixer bowl, beat the egg whites and granulated sugar using the whip attachment until they hold stiff peaks; fold into the cocoa mixture. Spread in a 9-by-13- inch pan and bake until the cake is lightly golden and springs back to the touch, about 12 minutes. Let cool on the baking sheet. Once cool, cut rounds slightly smaller than the base of the hazelnut sugar crust rounds.
For the blackberry cream: Soak the sheets of gelatin in a bowl of cold water, making sure they are completely covered, 5 minutes.
Meanwhile, heat the blackberry puree, butter, eggs and sugar until it registers 194 degrees F on a candy thermometer, then immediately remove from the heat. Whip the heavy cream in a bowl with an electric mixer until stiff peaks form.
Squeeze the water out of the gelatin and add the gelatin to the warm blackberry mixture, stirring to dissolve. Let cool to 90 degrees F, then fold in the whipped cream. Transfer to a pastry bag and pipe into small, round silicone molds. Place a hazelnut crust on top of each. (It should be smaller than the base of the cream to fit just inside the mold.)
For the chestnut cream: Soak the gelatin in a bowl of cold water for 5 minutes. Combine the milk, egg yolks and granulated sugar in a saucepan and cook over medium-low heat, stirring constantly with a rubber spatula, until it registers 165 degrees F and is thickened slightly. (Be careful not to exceed 165 degrees F or the custard may curdle.)
Squeeze the water out of the gelatin and add those sheets to the custard, stirring to dissolve. Add to a mixer fitted with the paddle attachment, then add the chestnut paste and cream and mix until combined. Chill, then transfer to a pastry bag.
Unmold the blackberry domes and place them on top of the sacher sponge rounds. Pipe chestnut cream on top of the blackberry domes. Place toasted hazelnuts and chopped chestnuts on top and serve.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.