Preparation

1. Grease and prepare your cake pans. Preheat the oven to 350°F (175°C).
2. In a large bowl, beat together the butter and sugar until creamy.
3. Add the egg whites and vanilla, beating until combined.
4. In another bowl, combine the cake flour and sour cream, alternating with the oil.
5. Mix just until combined, taking care not to overmix.
6. Gently fold in the sprinkles.

Baking

1. Pour the batter into prepared cake pans.
2. Bake for 25-32 minutes for 6-inch pans or 28-38 minutes for 8-inch pans, until a toothpick comes out clean.
3. Let the cakes cool in the pans for 10 minutes, then turn out onto a rack to cool completely.

Frosting

1. While the cakes cool, make the vanilla buttercream by beating the butter until pale and fluffy.
2. Gradually add powdered sugar while mixing on low speed.
3. Add vanilla, salt, and heavy cream until smooth.

Assembly

1. Once the cakes are completely cool, place one layer on a cake board, spreading a layer of buttercream on top.
2. Add the second layer and apply a crumb coat, chilling for 15 minutes.
3. Finally, apply the final coat of frosting and decorate with more sprinkles as desired.