First, make the sponge cake. Pre-heat the oven to 350 and line a jelly roll pan with parchment paper. Spray it liberally with cooking spray. Combine the powdered sugar and eggs in a large mixing bowl. Using a hand mixer, whip it together until it is pale, thick and fluffy for about 5 minutes. Slowly add in the flour just until it is mixed in. Be careful not to over mix it. Pour the batter into the prepared pan and smooth it out to be level. Bake the cake for about 20 minutes, until puffy and golden. When it is done, let it cool in the pan for a few minutes before turning it out onto a rack to finish cooling. Let the cake cool completely.

While the cake bakes and cools, prepare the espresso mixture for the sponge cake. Bring the water to a boil in a small pot on the stove. Dissolve the instant espresso in the boiling water and add the cognac. Give it a stir and then take the pot off of the heat. Set it aside to let it cool. Then prepare the mascarpone frosting. Set up a stand mixer with the paddle attachment and add the mascarpone to the bowl. Beat the mascarpone until fluffy, then add in the heavy cream. Lastly, add in the 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 the frosting aside.

Transfer the cooled cake to a large cutting board. Take the cake and trim the edges away from each side to make clean edges. Cut the cake in half through 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. 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.

Cover the cake with a dome and let it set in the refrigerator for at least an hour. Then slice and serve immediately!