• Preheat oven to 350º F.
• Spray three round 8 inch cake pans with cooking spray and line the bottoms with parchment paper. Wrap each pan with a damp cake strip (optional).

Make the cake batter.

• In a medium bowl, whisk together flour, baking powder, and salt. Set aside.
  2 ½ cups cake flour, 2 ¼ teaspoons baking powder, 1 teaspoon fine sea salt
• In the bowl of a stand mixer (or in a large bowl using an electric hand mixer), cream the butter and sugar for 3 minutes until blended and smooth.
  ½ cup unsalted butter, 1 ⅔ cups granulated sugar
• Add eggs and mix on low speed until blended. Then turn mixer up to high speed and beat for one minute.
  3 large eggs
• Add oil and both vanillas and beat until well blended.
  ¼ cup vegetable oil, 1 Tablespoon vanilla extract, 1 teaspoon vanilla bean paste
• With the mixer running on low speed, add one third of the flour mixture to the wet ingredients, stirring until blended. Add half of the buttermilk, continuing to stir until combined. Add another third of the flour mixture, followed by the remaining buttermilk, and then the final third of flour mixture, blending slowly between each addition.
  1 cup buttermilk
• Fold the sprinkles into the cake batter, being careful not to over mix the batter.
  ½ cup rainbow jimmies (sprinkles)

Bake the cake.

• Divide the batter evenly between the three prepared pans.
• Bake the cakes for 25-28 minutes (or 23-25 minutes if not using cake strips), or until the top sprigs back when gently poked and a cake tester inserted into the center of the cake comes out clean or with just a few moist crumbs.
• Run a cake tester or knife around the edge of the cake to keep it from sticking to the sides of the pan once cooled. Let the cake cool completely in the pan on a wire rack.

Make buttercream frosting.

• In the bowl of a stand mixer using the paddle attachment (or in a large bowl using a handheld electric mixer), beat butter on medium-high speed until smooth. 
  2 cups unsalted butter
• Add vanilla and salt and beat on medium-high speed until blended.
  4 teaspoons vanilla extract, ⅛ teaspoon fine sea salt
• Add sugar slowly – one cup at a time – beating well on medium speed and scraping the bowl often. When all sugar is added the frosting will be slightly dry. 
  8 cups confectioners' sugar
• Add milk and beat on high speed until the frosting is light and fluffy.
  ¼ cup whole milk

Assemble the cake.

• Place one cake layer onto a cake plate and spread frosting on top of the cake layer using an offset spatula.
• Place another cake layer on top of the first and spread frosting on top of the second layer.
• Place the third cake layer on top of the second layer.
• Spread a thin crumb coat of frosting on the sides and top of the cake. Chill the cake in the fridge for 5 minutes to firm up the frosting slightly.
• Spread the remaining frosting onto the sides and top of the cake.
• Decorate the cake with sprinkles.