A hazelnut cake layered with a white chocolate sour cream ganache and topped with fresh raspberries and white chocolate shavings.
White chocolate sour cream ganache
White chocolate sour cream ganache: In a 4-cup glass measuring cup or a large glass mixing bowl, melt white chocolate in the microwave on high for 1 minute. Whisk well and repeat microwaving and whisking until chocolate has melted completely, about 3 minutes. Pour in sour cream and whisk until well combined, then whisk in 1 cup of the whipping cream. Microwave the ganache on high heat for 4 to 5 minutes, until bubbly, then transfer it to a large bowl and whisk in the remaining 2 cups whipping cream until the mixture is thoroughly combined. Allow the ganache to cool to room temperature, cover with plastic wrap and refrigerate it for at least 8 hours.
Hazelnut cake: Preheat the oven to 350°F. Line two 8-inch cake or springform pans with parchment paper and grease the insides with nonstick baking spray.
In the bowl of a food processor, pulse hazelnuts with the baking powder and cornstarch until the nuts are finely chopped and the mixture is slightly coarse, not powdery. Set aside.
In a large bowl of a stand mixer, beat 1/2 cup of the sugar and the egg yolks on high until the batter falls gently from the whisk in ribbons and the eggs are thick and a light lemon colour, about 3 minutes. Set aside.
In a second bowl, whip egg whites on high until they are slightly foamy, about 1 minute. With the motor running, gradually add the remaining 1/4 cup sugar and beat until the whites form soft peaks, 1 to 2 minutes.
Using a spatula, gently fold 1/2 of the hazelnut mixture into the yolk and sugar mixture. When the mixtures are just combined, gently fold in the remaining hazelnut mixture. Next, fold in 1/2 of the egg whites until just combined, then gently fold in the remaining egg whites. Divide the batter equally between the two pans. (You should have about 3 cups of batter in each one.) Bake for 25 to 30 minutes (checking after 15 minutes), until the cakes are puffy and a knife inserted in the middle of each one comes out clean. Remove from the oven and allow to cool in the pans.
To assemble: Gently run a sharp knife along the edge of each pan to loosen the cake. If you are using springform pans, release the top of the pan. To remove the parchment paper, place a plate on top of each cake. Place one hand on the plate atop the cake, and, with the other hand, lift the cake pan (or springform base) and gently invert the cake onto the plate. Place the cake on the counter and carefully peel the parchment paper from the bottom of each cake. Discard the parchment paper. Set the cakes aside.
Taste the chilled ganache. If you find it too rich, add 1/4 cup whipping cream. (This is a case where adding more cream helps make it less rich.) In the bowl of a stand mixer, beat the ganache (and extra cream, if desired) on high, until it is thicker than whipped cream and just stiff enough to spread, 2 to 3 minutes. (Take care not to overbeat the ganache or it will separate and become grainy.)
Arrange one of the cakes, parchment-baked side up, on a serving platter. Using a spatula, spread 1/2 of the whipped ganache on the top of the cake and cover with a layer of raspberries. Set the other cake, parchment-baked side up, on top. Spread the remaining ganache on top and cover with more raspberries. Garnish with white chocolate shavings and dust lightly with icing sugar. Serve slightly chilled or at room temperature.