Preheat the oven to 350°F (175°C). Grease and flour a 9-inch round cake pan. In a large mixing bowl, cream together the unsalted butter and sugar until light and fluffy using an electric mixer.

Add the egg whites to the butter mixture and beat until well combined. In a separate bowl, whisk together the all-purpose flour, baking powder, salt, and cocoa powder.

In another small bowl, combine the milk, sour cream, water, and vanilla extract. Gradually add the dry ingredients to the wet ingredients, alternating with the milk mixture, mixing until smooth and well combined.

Pour the batter into the prepared cake pan and smooth the top. Bake in the preheated oven for 25-30 minutes or until a toothpick inserted into the center comes out clean.

Let the cake cool completely on a wire rack.

In a medium saucepan, heat the heavy cream and sugar over medium heat until the sugar dissolves and the cream begins to simmer.

In a separate bowl, whisk the egg yolks until they become slightly thickened. Gradually pour the hot cream mixture into the egg yolks, whisking constantly to temper the eggs.

Return the mixture to the saucepan and cook over low heat, stirring continuously until it thickens slightly. Remove from heat and stir in the semi-sweet chocolate chips. Let it melt completely, then set aside to cool.

Once cooled, whip the remaining heavy cream in a separate bowl until stiff peaks form. Gently fold the whipped cream into the cooled chocolate mixture until smooth and fully combined.

In a small bowl, dissolve the powdered gelatin in water. Let it sit for 5 minutes to bloom. In a heatproof bowl, melt the white chocolate using a microwave or a double boiler until smooth.

In a saucepan, heat 1 1/2 cups heavy cream and powdered sugar until it begins to simmer. Remove from heat, then add the bloomed gelatin and vanilla extract to the warm cream. Stir until completely dissolved.

Let the mixture cool slightly, then fold in the melted white chocolate. Whip the remaining heavy cream until stiff peaks form, then fold it gently into the white chocolate mixture until smooth.

Once the chocolate cake has completely cooled, slice it horizontally into two layers if desired. Place the bottom layer of cake onto a serving plate or cake stand. Spread the chocolate mousse evenly over the cake layer, smoothing it out with a spatula.

Add the second layer of cake and spread the white chocolate mousse on top. If you wish, garnish with additional chocolate shavings or decorative toppings. Chill the cake in the fridge for at least 3-4 hours or overnight to set the mousse layers.

Slice and serve. Enjoy!