Preheat the oven to 350 F. Lightly grease 9 or 10 inch springform and line the bottom with parchment paper. The cake should be baked in a water bath so the edges don’t dry out (wrap the springform pan in two layers of aluminum foil and place it in larger pan with about 1-1/2 inches of hot water).
Melt 10 oz. high-quality semisweet chocolate on low heat over a double boiler or in the microwave.
Place egg whites in a large bowl with about a third of the sugar and beating on low speed. Gradually increase the speed and beat until the mixture begin to look fluffy, then add the remaining sugar, salt and vanilla. Continue beating until all sugar is dissolved and the meringue looks shiny (nearly soft-peak stage).
Using a whisk stir softened butter into the melted chocolate until it’s combined, then whisk in the egg yolks. Add half of the meringue into the chocolate mixture and whisk gently, then with a spatula fold in the remaining meringue (keep in mind to mix in well the heavier batter at the bottom of the bowl)
Pour the batter into the prepared pan and bake for 26-28 minutes (Until the toothpick inserted into the center comes out clean).
When you remove the cake from the oven first let it cool to room temperature and then refrigerate it for 1 hour(cooled cake will sink a little and lose some volume). Take it out the fridge and run a knife around the edge of the pan, unlock the ring from the springform pan, take it off and invert the cake onto a serving plate. Remove the bottom of the pan and parchment paper then place back the ring from the springform around the cake to use it as a mold for the chocolate mousses.
Chocolate Mousse Layer:
Place 10 oz. high-quality semisweet chocolate(chopped) in a heat-proof bowl.
In a small dish soften 1 teaspoon unflavored gelatin in 2 tablespoons cool water set aside.
Bring 1/2 cup heavy whipping cream to a boil and pour it over the chopped chocolate, stir it gently, until smooth and the chocolate is completely melted. If the chocolate won’t melt completely re-heat the mixture on a low heat, and continue to stir until it’s all melted.
Heat softened gelatine on low heat, stirring to dissolve. When it’ s completely free of lumps, stir melted gelatin into the melted chocolate.
Whip the remaining 1 cup of heavy whipping cream(DO NOT overwhip it!!!) until a soft peaks form.
Chocolate mixture must be free of lumps and the temperature should be about 80 degrees F(in order to avoid that chocolate sets up, but if it’s too hot , whipped cream will melt).
Add about half of the whipped cream to the chocolate and whisk gently until it’s all combine, then fold in the remaining whipped cream (make sure to evenly incorporate all melted chocolate from the bottom of the bowl)
Spread the mousse over the top of the cooled cake in the ring and smooth it with the spatula. Place the cake back to the fridge.
White Chocolate Mousse Layer:
Repeat the directions for the chocolate mousse layer, using white chocolate(be careful not to overheat the white chocolate while it’s melting or it will separate and become unusuable)
Spread the white chocolate mousse over the top of the chocolate mousse and set back into the fridge for at least 4-5 hours (or overnight).
Before serving, run a thin knife around the edge, open the lock and lift off the springform pan ring.
Garnish the cake with chocolate curls or shaving if desired.