Recipe: Hazelnut Chocolate Cake with Mousse Filling and Espresso Buttercream

This decadent chocolate hazelnut cake is certainly a project for special occasions, but the end result makes it well worth the time and effort. Velvety layers of chocolate hazelnut cake marry with luscious chocolate hazelnut mousse, while toasted hazelnuts add crunch and a final layer of espresso buttercream adds a complimentary dimension of flavor.

Hazelnut Chocolate Cake
Adapted from Serious Eats
Baking spray and parchment paper
1 3/4 cups all-purpose flour
1 cup packed dark brown sugar
1 cup sugar
1 cup unsweetened cocoa powder
1 1/2 teaspoons baking powder
1 1/2 teaspoons baking soda
1 teaspoon salt
2 large eggs, at room temperature
1 cup sour cream, at room temperature
1/2 cup vegetable oil
1 tablespoon pure vanilla extract
1/4 cup hazelnut liqueur (Frangelico)
3/4 cup boiling water
2 teaspoons instant espresso powder
4 ounces bittersweet chocolate (60% cacao), finely chopped

Adjust oven rack to middle position and preheat oven to 350°F. Lightly spray two 9-inch round cake pans with baking spray. Line pans with parchment paper rounds and spray once again with baking spray.

In large bowl, whisk together flour, brown sugar, sugar, cocoa powder, baking powder, baking soda, and salt. In medium bowl, whisk together eggs, sour cream, oil, and vanilla. Add egg mixture to flour mixture and stir until mixture is a thick paste.
Stir espresso powder into hazelnut liqueur, then stir in boiling water, and whisk mixture into cake batter until smooth.

Heat water in a double boiler until simmering, and melt chocolate on top. Or, melt chocolate in microwave: microwave for 1 to 2 minutes, stopping and stirring every 30 seconds, just until melted. Mix melted chocolate into batter.

Scrape batter into prepared pans. Bake until cake tester inserted in center of cakes comes out clean, 20 to 25 minutes. Transfer cakes to cooling rack and cool in pans 10 minutes. Turn cakes out directly onto racks, peel off and discard parchment, then invert cakes so tops are facing up. Cool completely, about 1 hour.

Hazelnut Chocolate Mousse
Adapted from Gourmet
1 1/2 teaspoons unflavored gelatin
4 1/2 tablespoons cold water
3/4 cup chocolate hazelnut spread such as Nutella
3/4 cup mascarpone
2 1/4 cups chilled heavy cream
3 tablespoons unsweetened Dutch-process cocoa powder
4 1/2 tablespoons granulated sugar

Sprinkle gelatin over water in a 1- to 1 1/2-quart heavy saucepan and let stand until softened, about 5 minutes. Heat gelatin mixture over low heat, stirring, just until gelatin is melted, about 2 minutes. Whisk in chocolate hazelnut spread until combined and remove from heat.

Whisk together mascarpone and chocolate hazelnut mixture in a large bowl. Beat together cream, cocoa powder, and sugar in another large bowl with an electric mixer at low speed until just combined, then increase speed to high and beat until cream just holds soft peaks. Whisk one third of whipped cream into mascarpone mixture to lighten, then fold in remaining whipped cream until well combined.

Cover bowl with plastic wrap, pressing the plastic to the surface of the mousse, and refrigerate until slightly set, 1 to 2 hours.

Espresso Buttercream
Adapted from Delish.com
4 1/2 teaspoon instant espresso powder
6 tsp. vanilla extract
9 sticks unsalted butter, at room temperature
7 1/2 cups powdered sugar, sifted

In a small bowl, combine espresso powder and vanilla extract and stir with a fork until espresso is dissolved.

In the bowl of a stand mixer, beat butter until smooth. Scrape down sides of bowl and add sugar, no more than 1 cup at a time, beating first on low and increasing mixer speed to high. Scrape down sides of bowl and continue adding sugar and mixing until frosting is smooth. Add espresso mixture and continue to beat until frosting looks light and fluffy, 1 to 2 minutes more.

Store at room temperature while you are assembling the cake.

Assembly
I cup whole blanched hazelnuts
1/4 cup hazelnut liqueur
Disposable pastry bags
Pastry brush
Offset icing spatula

Preheat oven to 350°F. Place hazelnuts on a rimmed baking sheet and toast until fragrant and light golden brown, about 15 minutes. Remove from oven and let cool. Once cool, roughly chop and set aside.

Divide the hazelnut chocolate mousse evenly between 3 disposable pastry bags. Cut each cake round in half horizontally with a serrated knife, making 4 equal cake layers. Place one cake round on desired serving plate and brush top all over with hazelnut liqueur. Take one of the filled pastry bags, cut off the tip, and pipe mousse evenly over surface of cake. Using the icing spatula, smooth mousse so that it is evenly distributed over the layer. Sprinkle 1/3 of chopped toasted hazelnuts over layer. Place the second cake layer on top, and repeat the process with the remaining 2 pastry bags of mousse, hazelnuts, and cake layers. You will have a 4-layer cake with 3 layers of mousse and hazelnuts sandwiched in between. Place cake in the refrigerator and chill for at least one hour.

After it has chilled, take the cake out and apply the crumb coat of frosting. Using the offset icing spatula, spread a very thin layer of the espresso buttercream all over the cake, making sure the top of the cake is level and the sides are perfectly vertical. This layer of icing should be so thin, that some of the cake is still visible underneath (refer to the picture below). Keep remaining buttercream out at room temperature, but place the cake back in the refrigerator to chill for at least one hour.

The crumb coat of frosting should be thin, leaving some cake bare.
The crumb coat of frosting should be thin, leaving some cake bare.

After it has chilled, remove the cake from the refrigerator and apply the final coat of frosting. Again using the offset spatula, apply buttercream evenly all over the sides and top of the cake. Rinse the spatula in hot water as you are doing this so the frosting doesn’t drag or clump up as you work. Decorate as desired, using any leftover buttercream to pipe designs or simply garnishing with whole or chocolate-covered hazelnuts.

-Vera Kirillov

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