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 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 3-5 minutes, then remove to cooling racks to cool completely.
9. While the cakes cool, make the filling. To make the filling, add all the ingredients to a mixer bowl and mix on low speed until well combined. Do not overmix or mix on a high speed or the mascarpone cheese will become thin and watery. If you are using a ricotta cheese that is watery, you may want to strain it first.
10. Place the filling in the fridge to firm up a bit, about 2-3 hours.
11. When the cakes are cool and the filling is ready, build the cake. First, remove the domes from the tops of the cake layers with a large serrated knife.
12. Place the first layer of cake on a cake circle or serving stand, then spread about 1 1/2 cups of filling on top. It should have firmed up in the fridge, but might still be a bit soft, so work relatively quickly.
13. Repeat with the next layer of cake and filling, then top the cake with the final layer of cake.
14. Use any remaining filling to quickly add a crumb coat to the cake, then place the cake in the fridge to firm up, 2-3 hours. If you have a large cake carrier to store it in, that’s best.
15. While the cake firms up, make the frosting. To make the frosting, add all the ingredients to a mixer bowl and mix on low speed until well combined. Do not overmix or mix on a high speed or the mascarpone cheese will become thin and watery.
16. Place the frosting in the fridge to firm up a bit, about 2-3 hours.
17. When the cake and frosting are ready, frost the outside of the cake. I used my offset spatula to get a nice, even layer. Keep aside about 1/2 – 3/4 cup of frosting for the border.
18. Press the mini chocolate chips onto the sides of the cake.
19. Pipe a border around the top edge of the cake. I used Ateco tip 844, but Wilton 2D or 1M would work as well.
20. Refrigerate cake until ready to serve.