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

In a large bowl, whisk together the cake flour, baking powder, baking soda, and salt. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and granulated sugar together on medium speed until light and fluffy, about 3 to 4 minutes.

Add the vegetable oil and mix for an additional 30 seconds. Then add the eggs one at a time, mixing well after each addition.

Mix in the vanilla extract until fully incorporated.

Gradually add the dry ingredients into the wet mixture, alternating with the buttermilk and sour cream, starting and ending with the dry ingredients. Mix until just combined.

Fold in the rainbow jimmies carefully.

Divide the batter between the prepared pans and smooth the tops.

Bake in the preheated oven for about 25 to 35 minutes, or until a toothpick inserted in the center comes out clean.

Let the cakes cool in the pans for 10 minutes. Run a knife around the edges, then invert onto a wire rack to cool completely.

In a clean mixer bowl, beat the butter on medium speed until creamy.

Gradually add the sifted powdered sugar one cup at a time, mixing on low to avoid a sugar cloud. Scrape the sides of the bowl as needed.

Add salt and vanilla extract, beating on medium-high until light and creamy, about 3 minutes.

Add whole milk, one tablespoon at a time, until you reach a good spreadable consistency.

Trim the cake layers if necessary. Place one cake layer on a cake board or serving plate and spread buttercream on top.

Stack the remaining layers, adding buttercream in between each.

Apply a crumb coat around the cake and chill in the fridge for 20 to 30 minutes.

Finish with a smooth outer layer of buttercream, decorating with extra sprinkles if desired.