Prep Time: 1 hour
Cook Time: 50 minutes
Total Time: 1 hour, 50 minutes
- 1 pound hazelnuts
- 4 tablespoons butter, softened
- 2 ounces unsweetened chocolate, broken into 1/2-ounce pieces
- 4 tablespoons plus 1 teaspoon unsalted butter, softened
- 1 cup plus 1 teaspoon cake flour
- 1/4 cup unsweetened cocoa
- 1 teaspoon baking soda
- 1/2 teaspoon salt
- 1 cup plus 4 tablespoons, packed light brown sugar
- 2 eggs
- 1/2 teaspoon pure vanilla extract
- 1/2 cup water
- 3/4 cup sour cream
- 1-3/4 cups heavy cream
- 2 tablespoons sugar
- 2 tablespoons unsalted butter
- 14 ounces semisweet chocolate, broken into 1/2-ounce pieces
For the hazelnut butter: toast and skin the hazelnuts, then finely chop them in a food processor fitted with a metal blade. Add 4 tablespoons butter and process until combined. Set aside.
---For the cake: Raise the oven temperature to 350 degrees F. Heat 1 inch of water in the bottom half of a double boiler over medium heat. Place the unsweetened chocolate in the top half of the double boiler. Tightly cover the top with film wrap, heat for 4 to 5 minutes. Remove from heat and stir until smooth.
Coat a 9- by 1-1/2 inch cake pan with 1 teaspoon butter. Flour the pan with 1 teaspoon flour, shaking out the excess. Sift 1 cup flour, the cocoa, baking soda, and salt onto wax paper. Set aside.
Combine the brown sugar and 4 tablespoons butter in the bowl of an electric mixer fitted with a paddle. Beat on low for 2 minutes. Scrape down the sides of the bowl, then beat on high for 1-1/2 minutes. Scrape again. Add 1 egg and beat on high for 15 seconds, scrape again. Add the remaining egg, beat on high for 15 seconds. Scrape one more time. Beat on high for 2 minutes. Add the melted chocolate and the vanilla and beat on low for 30 seconds. Scrape down the bowl one more time.
Heat 1/2 cup water to a boil in a small saucepan. With the mixer on low, add a third of the sifted flour and half the sour cream to the chocolate mixture and mix for 30 seconds. Add the next third of flour and the rest of the sour cream and mix again for 30 seconds. Add remaining flour and the water and mix for 30 seconds. Use a rubber spatula to finish smoothing out the cake batter.
Spread cake batter into evenly into prepared pan. Bake in the preheated oven until a toothpick inserted in the center of the cake comes out clean, 45 to 50 minutes. Cool cake in the pan for 15 minutes at room temperature. Turn out onto a cooling rack and keep at room temperature until needed.
---For the ganache: Heat the heavy cream, 2 tablespoons sugar, and 2 tablespoons butter in a 2-1/2 quart saucepan over medium-high heat, stirring to dissolve the sugar. Bring the mixture to a boil. Place the semisweet chocolate in a stainless steel bowl. Pour the boiling cream over the chocolate and allow to stand for 5 minutes. Stir until smooth. Keep at room temperature until ready to use.
Slice cake horizontally into 3 equal layers. Place a dab of ganache in the center of a cake circle. Place the top layer of the cake, cut side down, onto the circle, gently pressing down on the cake so that the ganache will hold it in place. Place the center layer of the cake on a separate cake circle. Also place the bottom layer of the cake, cut side down on a third cake circle. Evenly spread 3/4 cup of the ganache over the top and center layers of the cake. Refrigerate these layers for 20 minutes. Remove the 2 layers form the refrigerator and divide and spread the hazelnut butter over the ganache on both cake layers, spreading evenly to the edges. Refrigerate for 20 minutes. slide the ganache center layer onto the ganache top layer. Slide the bottom layer onto ganache center layer. Pour the remaining ganache over the cake, spreading with a spatula to create and even coating of ganache on both the top and sides of the cake. Refrigerate the cake for at least 2 hours before cutting and serving.
Cut the cake with a serrated slicer, heating the blade of the slicer under hot running water before making each slice. Allow the slices to come to room temperature for 30 to 40 minutes before serving.