Preheat the oven to 350°F. Lightly grease a 9 or 10 inch springform 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. 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 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.

Add half of the meringue into the chocolate mixture and whisk gently, then fold in the remaining meringue with a spatula.

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

Cool the cake to room temperature, then refrigerate for 1 hour. Run a knife around the edge, unlock the ring from the springform pan, invert the cake onto a serving plate, and remove the bottom of the pan and parchment paper. Place the ring back around the cake.

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

Bring 1/2 cup heavy whipping cream to a boil and pour it over the chopped chocolate, stirring until smooth. Reheat on low if necessary until fully melted.

Heat softened gelatin on low heat, stirring to dissolve. Stir melted gelatin into the melted chocolate.

Whip the remaining 1 cup of heavy whipping cream until soft peaks form.

Combine half of the whipped cream with the chocolate mixture, then fold in the remaining whipped cream.

Spread the chocolate mousse over the top of the cooled cake and smooth with a spatula. Return the cake to the fridge.

For the white chocolate mousse layer, repeat the process using the 7 oz. high-quality white chocolate.

Spread the white chocolate mousse over the chocolate mousse layer 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 or freeze for up to several weeks.