Perfect Birthday Cake

Make cake: Preheat oven to 350° and line three 8" cake pans with parchment paper. In a large bowl using hand mixer (or in the bowl of a stand mixer), beat together butter and sugar until light and fluffy. Add eggs one at a time, beating well after each addition. Add vanilla and mix until combined. In another large bowl, whisk together flour, cornstarch, baking powder, and salt. Add half of the dry ingredients to the wet ingredients, beating until just combined. Pour in the milk and mix until fully incorporated. Add remaining dry ingredients and stir until just combined. Fold in sprinkles. Divide batter evenly between prepared cake pans. Bake until slightly golden on top and a toothpick inserted into the middle comes out clean, about 30 to 35 minutes. Let cool completely. Meanwhile, make buttercream: in a large bowl using a hand mixer, beat butter until light and fluffy, 2 to 3 minutes. Add powdered sugar, sour cream, vanilla, heavy cream, and salt and beat until smooth. Assemble cake: Place the bottom layer of the cake on your serving platter and spread with buttercream; repeat with the second layer of cake. Place the last layer of cake on top, then frost the entire cake with buttercream. Garnish with more sprinkles, if desired.