Preheat the oven to 350°F/180°C. Spray two 9-inch round cake pans with baking spray and line with parchment cake rounds, set aside.
In a large bowl, whisk together the cake flour, baking powder, baking soda, and salt, set aside.
In the body of a stand mixer, add the egg whites with the whisk attachment, beat until light and fluffy and medium peaks are formed about 3 minutes. Take the egg whites out and set aside.
No need to clean out the bowl, cream the butter with the sugar until light pale yellow, light and fluffy, about 3 minutes with the paddle attachment. Add the sour cream and vanilla, mix until combined.
Add the flour mixture and stir in until combined, scrape down the sides. Add in the heavy cream and stir in until combined, scrape the sides. Fold in the egg whites. Add in ¾ cup of the sprinkles and fold them in.
Pour the batter evenly into the two cake pans, smooth out the tops. Bake for 26-32 minutes, until a toothpick inserted into the center, comes out mostly clean (To make sure the cakes are done, you can take an internal temperature, if the cakes are between 205°-210°F (96-99°C), they are done.). Let the cakes cool completely in the pans on a wire rack.
Make the buttercream frosting: In a large bowl using a hand mixer, or using a stand mixer that's fitted with the paddle attachment. Beat the butter at medium speed for 2 minutes until creamy. Decrease the speed to low, and add the powdered sugar, cream, vanilla extract, and salt and mix again. When the sugar is mixed in, increase the speed to high and beat for 3 minutes. If the frosting is too thick, add cream and beat again. If it's too thin, add powdered sugar and beat again.
Crumb Coat: Once the cakes are completely cool, take them out of the pans. Place one of the cakes bottom side down on your serving platter or cardboard cake board. Add 1 cup of buttercream frosting on top and smooth it out. Invert the other cake so the bottom side is up, add it on top of the frosting. Crumb coat the cake all over. Place in the fridge for at least two hours so the outside layer of frosting sets. I like to do this step then let it chill overnight.
Decorate: When ready to decorate, add a thick layer of buttercream around the sides and the top of the cake, make it as even as possible and smooth it out as best you can. Don’t worry if the sides are messy, we are about to cover that up! Taking the remaining sprinkles, gently press them in all around the bottom edge of the cake. Decorate the top of the cake however you wish and decorate with more sprinkles if desired. I like to use an open star tip to pipe my decoration on top.