1. Preheat oven to 350°F (176°C) and prepare an 8 inch cake pan with parchment paper in the bottom and grease the sides.
2. In a large mixing bowl, cream the butter and sugar together until light in color and fluffy, about 3-4 minutes. Do not skimp on the amount of creaming time.
3. Add sour cream and vanilla extract mix until well combined.
4. Add egg whites one at a time, mixing until well combined after each. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.
5. Combine dry ingredients in a separate bowl, then combine milk and water in a small measuring cup.
6. Add half of the dry ingredients to the batter and mix until well combined. Add the milk mixture and mix until well combined. Add remaining dry ingredients and mix until well combined.
7. Spread the batter evenly into the prepared pan and bake for 19-21 minutes, or until a toothpick inserted in the middle comes out with a few crumbs.
8. Remove cakes from oven and allow to cool for 3-4 minutes, then remove to cooling rack to cool completely.
9. One the cake is cooled, prepare the cake pan for layering. To set up your cake pan, line an 8×3 inch springform pan with parchment paper that goes about 2 inches above the top edge of the pan. A springform pan is best so that you can easily remove the cake once it’s been assembled. If you only have a regular 8×3 inch cake pan, line the cake pan with plastic wrap (which can later be used to lift your cake out of the pan once it’s assembled), then add the parchment paper. You can also place a cardboard cake circle in the bottom of the pan, on top of the clear wrap. In lou of using a cake pan to build the cake, you could also use cake collars.
10. Cut the dome off of the cake, then place it in your pan or cake collar.
11. To make the fudge layer, add the chocolate chips, corn syrup and vanilla extract to a medium heat-proof bowl.
12. Heat the heavy whipping cream until it just starts to boil, then pour it over the chocolate chips. Cover the bowl with clear wrap and allow to sit for 3-5 minutes, then whisk until smooth.
13. Pour the chocolate on top of the cake and spread into an even layer. Place the cake in the fridge while the ganache firms, about 1 hour.

TO MAKE THE CHOCOLATE MOUSSE:

1. Combine egg yolks, sugar and 1/2 cup (120ml) of heavy whipping cream in the top of a double boiler. If you don’t have a double boiler, you can use a mixing bowl set over a pot with simmering water in it. I use my mixer bowl. Do not let the water boil, or it will be too hot. To ensure that the steam cooking the eggs doesn’t get too hot, occasionally lift the bowl to release the steam.
2. Cook mixture, whisking constantly. The mixture will thin out at first, then begin to thicken. The egg mixture will be ready when is has thickened, lightened in color and has more volume. It should take about 7-10 minutes and reach 160°F (71°C). Do not boil. When done, remove egg mixture from heat and set aside.
3. Place the chocolate chips for mousse in a medium sized bowl and melt in the microwave in intervals of about 10-15 seconds, stirring well between each interval, until smooth. You could also melt them in a double boiler, if you prefer.
4. Vigorously whisk the egg mixture into the melted chocolate until smooth. Set aside to cool to about room temperature.
5. In another bowl, whip remaining 1 1/4 cups (300ml) of heavy whipping cream and powdered sugar until stiff peaks form.
6. Fold about a ¼ cup of the whipped cream into the chocolate mixture until , then fold about a 1/4 of the chocolate mixture into the whipped cream. Slowly fold the remaining chocolate mixture into the whipped cream.
7. Spread the chocolate mousse into an even layer on top of the chocolate fudge. Refrigerate for about an hour.

TO MAKE THE WHITE CHOCOLATE MOUSSE:

1. In a small bowl, sprinkle the powdered gelatin over the water, making sure the powder is all touching water. Let stand for about 5 minutes.
2. Place the white chocolate chips in a medium glass bowl. Microwave 1/2 cup (120ml) of heavy cream in a medium measuring cup until it comes to a boil.
3. Add the gelatin mixture to the hot heavy cream mixture and whisk until the gelatin is dissolved.
4. Pour the hot cream over the white chocolate chips and let stand, covered, for 3-5 minutes. Whisk the white chocolate mixture until the chocolate is melted and the mixture is smooth. If it doesn’t completely melt, microwave in 10-15 second intervals, whisking well between each, until smooth. Set aside to cool to about room temperature.
5. Whip remaining 1 1/4 cups (300ml) heavy cream and powdered sugar at high speed with a mixer until stiff peaks form.
6. Gently fold about a third of the cooled white chocolate mixture into the whipped cream until combined. Add the second third of the cooled white chocolate and gently fold into the whipped cream. Add the remaining white chocolate and fold together until completely combined.
7. Spread the white chocolate mousse into an even layer on top of the chocolate mousse. Refrigerate until completely firm, at least 3-4 hours.

TO FINISH OFF THE CAKE:

1. Once firm, remove the cake from the pan or remove the cake collar from the sides. If needed, smooth the sides of the cake using an offset spatula, and place cake on a serving plate.
2. Finish off the cake with chocolate curls and a sprinkle of cocoa powder. Keep cake refrigerated until ready to serve.