3 cups cake flour
2 1/2 cups granulated sugar
1 tablespoon baking powder
1 teaspoon salt
1 1/2 cups (3 sticks) unsalted butter, softened and cut into cubes
1 1/4 cup whole buttermilk, divided
5 large eggs
2 teaspoons vanilla extract
1 cup confetti quin sprinkles
2 cups (4 sticks) unsalted butter, room temperature
3 3/4 cups powdered sugar
1/4 cup heavy cream, more or less as needed
1 vanilla bean, seeded
food coloring (optional)
Preheat oven to 350 degrees F. Butter the bottoms and sides of three 9-inch round cake pans. Line with parchment paper; butter parchment.
In a large mixing bowl or the bowl of a stand mixer, combine flour, sugar, baking powder, and salt.
Mix to combine. Add butter and 1 cup of buttermilk. Mix on low speed until flour is evenly moistened, then increase speed to medium-high and beat until light and fluffy, 1 to 2 minutes, scraping down the sides of the bowl as needed.
In a small bowl, beat remaining 1/4 cup buttermilk with eggs and vanilla. Add to the batter mixture one-third at a time, mixing well after each addition. Stir in sprinkles. Divide batter evenly among prepared pans.
Bake for 35 to 45 minutes (about 25 to 30 minutes for 6? pans), rotating pans halfway through baking, until tops are lightly golden on the edges and a toothpick inserted near the center comes out clean. Transfer pans to a wire cooling rack to cool completely, about 1 hour. Run a knife or thin spatula around the edge of the pan, then invert the cake onto wire rack.
It should come out easily and cleanly. Remove parchment from the bottom. At this point the cakes can be wrapped tightly in a double layer of plastic wrap and frozen until ready to use (in fact, I recommend you freeze the cakes overnight, as the frozen cakes will be easier to level and frost later.)
To prepare frosting, in a large mixing bowl, beat butter on medium-high speed until smooth and fluffy, 2 to 3 minutes. Add powdered sugar, 1/2 cup at a time, mixing well after each addition. As frosting thickens, add a few tablespoons of cream as needed.
Depending on the temperature and consistency of your frosting, continue alternating additions of cream and powdered sugar, until frosting is light and fluffy. Add vanilla bean seeds and food coloring and beat until evenly incorporated.
To assemble, level each cake layer by cutting of the domed top with a long serrated knife.
Place one layer, flat side down, on a cake stand or serving platter. Spread on about 1/2 cup of frosting onto cake using an offset spatula.
Position second layer on top and press to adhere. Repeat with another 1/2 cup of frosting, and then position final cake layer, flat side up.
Cover the entire cake with a thin layer of frosting. This “crumb coat” will keep the stray crumbs in place and make frosting the cake easier. Refrigerate for about 15 minutes to allow this crumb coat to set, then spread with remaining frosting, reserving some frosting for piping decorative details and borders if desired