1. Prepare three 8-inch cake pans with parchment paper circles in the bottom and grease the sides. Preheat oven to 350°F (176°C).
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 creaming time.
3. Add sour cream and vanilla extract and mix until well combined.
4. Add egg whites in two batches, 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 the 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. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.
7. Divide the batter evenly between the cakes pans and bake for about 21-23 minutes, or until a toothpick comes out with a few crumbs.
8. Remove the cakes from the oven and allow to cool for about 2-3 minutes, then remove to cooling racks to cool completely.

TO MAKE THE WHITE CHOCOLATE MOUSSE:

9. Heat 1/4 cup (60ml) heavy cream until it just starts to boil, then pour over the white chocolate chips. Cover with clear wrap for 3-4 minutes, then whisk until smooth. Sometimes the white chocolate doesn’t completely melt. If that happens, microwave white chocolate mixture in 10 second increments, stirring well between each increment, until smooth. Set aside to cool to about room temperature.
10. Whip remaining 3/4 cup (180ml) of heavy whipping cream and powdered sugar in a large mixer bowl fitted with the whisk attachment until stiff peaks form.
11. Carefully fold about 1/3 of the whipped cream into the cooled white chocolate mixture until combined.
12. Fold in the remaining whipped cream until well combined. Set the mousse in the refrigerator until ready to use.

TO MAKE THE RASPBERRY MOUSSE AND FILLING:

13. Puree raspberries with a food processor, then strain through a fine mesh sieve to discard seeds. You should end up with about 1 1/4 cups of puree.
14. To make the raspberry filling, add half a cup of the raspberry puree to a small saucepan, along with the cornstarch and sugar. Heat slowly over medium heat until the mixture begins to thicken and boil. Boil for one minute, then remove from heat to cool. Set aside.
15. To make the raspberry mousse, add remaining 3/4 cup of raspberry puree to a large dish, so that the puree makes a thin layer.
16. Sprinkle the powdered gelatin evenly over the puree and let stand for 4-5 minutes.
17. Heat the puree and gelatin in the microwave in 10-second intervals until warm and smooth. Set the mixture aside to cool to room temperature or in the fridge. Note that the colder the mixture gets, the firmer it will be. If you want a more cohesive pink color frosting, you won’t want it to firm up too much before adding to the whipped cream in the next steps, but it should be at least room temperature. The firmer the mixture gets, the chunkier your raspberry mixture will be.
18. Whip the heavy whipping cream and powdered sugar in a large mixer bowl fitted with the whisk attachment until stiff peaks form.
19. Add the cooled raspberry gelatin mixture to the whipped cream and gently whip until well combined. Refrigerate until ready to use. If your raspberry mixture was still a little thin when you added it, the whipped cream may be a bit thin. It should firm up in the fridge.

TO ASSEMBLE THE CAKE:

20. Use a large serrated knife to remove the domes from the top of the cakes so that they’re flat. Place the first cake on a serving plate or a cardboard cake round.
21. Pipe a dam of raspberry mousse around the edge of the cake, then spread 4-5 tablespoons of raspberry filling evenly into the center.
22. Add about half of the white chocolate mousse on top of the raspberry filling and spread into an even layer.
23. Add the second cake layer and another layer of raspberry filling and white chocolate mousse.
24. Top the cake with the remaining layer of cake.
25. Frost the outside of the cake. If needed, refer to my tutorial on how to frost a smooth cake.
26. Pipe swirls around the top of the cake. I used Ateco tip 808. Finish off the cake with some additional raspberries and shaved white chocolate. Refrigerate until ready to serve. Cake is best for 2-3 days.