Step 1: Make the Cake Batter

1. Preheat oven to 350°F (175°C). Grease and line two 9-inch cake pans with parchment paper.
2. In a bowl, whisk together flour, baking powder, and salt. Set aside.
3. In a large mixing bowl, beat butter and sugar until light and fluffy (2-3 minutes).
4. Add eggs one at a time, mixing well after each. Stir in vanilla extract.
5. Mix in sour cream until combined.
6. Gradually add flour mixture and milk, alternating between the two, mixing just until combined.
7. Gently fold in rainbow sprinkles (be careful not to overmix, or the colors may bleed).

Step 2: Bake the Cake

8. Divide the batter evenly between the prepared cake pans.
9. Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.
10. Let cakes cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.

Step 3: Make the Vanilla Buttercream Frosting

11. In a large bowl, beat butter until creamy (2 minutes).
12. Gradually add powdered sugar, beating well after each addition.
13. Mix in vanilla extract and a pinch of salt.
14. Add heavy cream and beat until fluffy and smooth (2-3 minutes).

Step 4: Assemble & Decorate

15. Place one cake layer on a plate and spread a layer of buttercream frosting on top.
16. Add the second cake layer and frost the top and sides evenly.
17. Decorate with extra sprinkles and cake toppers as desired.

Step 5: Serve & Enjoy!

18. Slice and serve with a big smile!