How To Make Chocolate Mousse Cake. You will need. 10 ounces bitter sweet chocolate, 9 tablespoons unsalted butter, 6 large eggs, separated, Pinch of salt, 1/4 cup sugar, 2 tablespoons brandy, 1 teaspoon confectioners sugar. Step 1. Preheat oven to 350 degrees. Cover the outside of a 9-inch spring-form pan with a double layer of foil. Using a microwave oven or double boiler, melt together chocolate and butter. Set aside to cool. Step 2. Using an electric mixer, whisk egg whites and salt until thick. Add 1/4cup granulated sugar, and continue to whisk until stiff and shiny but not dry. In another bowl, whisk together egg yolks and remaining 1/2 cup sugar until pale and thick. Whisk in brandy, then fold in cooled chocolate mixture. Step 3. Place a kettle of water over heat, and bring it to a boil. Fold about 1/2 cup whisked egg whites, being careful not to let the mixture deflate. Pour mixture into prepared spring-form pan, and place in a roasting pan. Add boiling water to roasting pan about halfway up side of spring-form pan. Bake for 45 minutes, top of cake will be hard and inside will be gooey. Step 4. Remove cake pan from water, and place on a rack to cool completely. unwrap foil and remove side of spring-form pan. Place cake on a serving platter. Just before serving, dust top with sifted confectioners sugar. Enjoy.