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For the sponge cake:
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Preheat oven to 350u00b0F and line a jelly roll pan with parchment paper. Spray liberally with cooking spray.
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Combine powdered sugar and eggs in a large mixing bowl. Using a hand mixer, whip until pale, thick and fluffy, about 5 minutes. Slowly add flour just until mixed in. Be careful not to over-mix.
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Pour batter into prepared pan and level. Bake cake for about 20 minutes, until puffy and golden. When done, let cool in the pan for a few minutes before turning it out onto a rack to finish cooling. Let cake cool completely.
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While cake bakes and cools, prepare the espresso mixture for the sponge cake. Bring water to a boil in a small pot on the stove. Dissolve instant espresso in the boiling water and add cognac. Stir and then take the pot off heat. Set it aside to cool.
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For the frosting:
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Set up a stand mixer with the paddle attachment and add the mascarpone to the bowl. Beat until fluffy, then add heavy cream. Lastly, add powdered sugar and nutmeg to bring it together. It will be a very wet frosting, and that's ok. The sponge will absorb it and it will set in the refrigerator once the cake is assembled. Set frosting aside.
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To assemble:
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Transfer cooled cake to a large cutting board. Trim edges away from each side to make clean edges. Cut cake in half across the short side, then cut each half in half lengthwise to get four equal rectangles. Place the first rectangle on a cake stand and spoon 1/4 of the espresso cognac mixture all over it. It will absorb right into the sponge. Then spread 1/4 of the frosting on the sponge and grate a generous layer of the chocolate right over it. Repeat the process with the next two rectangles of cake to make two more layers.
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Place the last rectangle on top and soak it with the remaining espresso liquid. Spread the last of the mascarpone frosting on in a smooth layer, then sift the cocoa powder in a layer on top. Using a vegetable peeler, peel pretty shavings of more chocolate on top. Then place three squares of chocolate on top for decoration.
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Cover the cake with a dome and let it set in the refrigerator for at least 1 hour. Then slice and serve immediately!
