Preheat the oven to 350 F. Lightly grease a 9 or 10 inch springform pan and line the bottom with parchment paper. Wrap the springform pan in two layers of aluminum foil and place it in a larger pan with about 1-1/2 inches of hot water.

Melt 10 oz. of high-quality semisweet chocolate on low heat over a double boiler or in the microwave.

In a large bowl, combine egg whites with a third of the sugar and beat on low speed. Gradually increase the speed and beat until fluffy, then add the remaining sugar, salt, and vanilla. Continue beating until the meringue is shiny and nearly at soft-peak stage.

Stir softened butter into the melted chocolate until combined, then whisk in the egg yolks. Gently fold in half of the meringue, then fold in the remaining meringue.

Pour the batter into the prepared pan and bake for 26-28 minutes, or until a toothpick inserted into the center comes out clean.

Let the cake cool to room temperature, then refrigerate for 1 hour.

Once cooled, run a knife around the edge of the pan, remove the springform ring, and invert the cake onto a serving plate. Remove the parchment paper and place the ring back around the cake to use as a mold.

For the chocolate mousse layer, place 10 oz. of high-quality semisweet chocolate in a heat-proof bowl. Soften 1 teaspoon of unflavored gelatin in 2 tablespoons of cool water and set aside.

Bring 1/2 cup of heavy whipping cream to a boil and pour it over the chopped chocolate, stirring until smooth. Reheat gently if chocolate does not melt completely.

Heat the softened gelatin on low heat, stirring to dissolve. Once smooth, stir the gelatin into the melted chocolate.

Whip 1 cup of heavy whipping cream until soft peaks form. Ensure the chocolate mixture is free of lumps and at about 80 degrees F, then add half of the whipped cream to the chocolate and whisk gently. Fold in the remaining whipped cream.

Spread the chocolate mousse layer over the top of the cooled cake in the ring and smooth it out. Place back in the fridge.

For the white chocolate mousse layer, repeat the directions for the chocolate mousse, using white chocolate. Spread the white chocolate mousse over the chocolate mousse and refrigerate for at least 4-5 hours or overnight.

Before serving, run a thin knife around the edge, unlock, and lift off the springform pan ring. Garnish with chocolate curls or shavings if desired. Store in the fridge.