Heat the oven to 350°F (175°C). Grease and flour two 9-inch round pans or three 8-inch pans. Line the bottoms with parchment and set aside.

Whisk the cake flour, baking powder, baking soda, and salt in a large bowl. Set aside to evenly distribute the leaveners.

In a stand mixer fitted with the paddle, beat the 12 tablespoons of butter and granulated sugar on medium-high until pale and fluffy, about 3 to 4 minutes.

Add the vegetable oil and beat for 30 seconds to combine. Add eggs one at a time, mixing well after each addition.

Stir in the vanilla extract.

With the mixer on low, add the dry ingredients in three parts, alternating with the buttermilk and sour cream. Start and finish with the dry ingredients, and mix just until combined.

Gently fold in the rainbow jimmies with a spatula, being careful not to overmix.

Divide the batter evenly among the prepared pans. Smooth the tops and bake until a toothpick comes out clean, about 25 to 35 minutes.

Let the cakes cool in their pans for 10 minutes, then turn them out onto a rack and cool completely.

For the buttercream, beat the 1.5 cups of butter on medium speed until creamy.

Gradually add the sifted powdered sugar, then add the salt and vanilla. Beat until light.

Add whole milk a tablespoon at a time until the buttercream reaches the desired spreadable consistency.

Trim the cake layers if needed for even stacks. Spread buttercream between layers, apply a thin crumb coat, and chill for 20 to 30 minutes.

Finish with a smooth outer coat of buttercream and press extra sprinkles on top or along the sides.