Step 1: Prepare the Cake Pans : Preheat your oven to 350°F (175°C). Grease and line two 9-inch round cake pans with parchment paper.

Step 2: Make the Cake Batter : In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside. In a large mixing bowl, cream butter and sugar together for 3–4 minutes until light and fluffy. Add eggs one at a time, beating well after each addition. Stir in vanilla extract. Mix in sour cream until combined. Alternate adding dry ingredients with milk, starting and ending with flour mixture. Mix until just combined — do not overmix. Gently fold in the rainbow sprinkles with a spatula until evenly distributed.

Step 3: Bake the Cake : Divide the batter evenly between prepared pans and smooth the tops. Bake for 28–32 minutes, or until a toothpick inserted in the center comes out clean. Let cakes cool in the pans for 10 minutes, then transfer to a wire rack to cool completely before frosting.

Step 4: Make the Vanilla Buttercream : In a stand mixer, beat the softened butter on medium speed for 3–4 minutes until creamy and pale. Gradually add powdered sugar, 1 cup at a time, mixing on low. Add vanilla extract, salt, and heavy cream. Beat on medium-high speed for 2–3 minutes until smooth, fluffy, and spreadable. Add more cream if needed for consistency.

Step 5: Assemble the Cake : Place one cooled cake layer on a serving plate. Spread a generous layer of buttercream on top. Add the second layer and cover the entire cake with buttercream using an offset spatula. Smooth the top and sides, then decorate with rainbow sprinkles. Chill for 30 minutes before slicing to help the cake set.