1-First Step: Preheat your oven to 350°F. Grease three 8-inch round cake pans thoroughly with butter or baking spray, then line the bottoms with parchment paper rounds. This preparation ensures the cakes will release easily after baking. Prepare your ingredients by bringing all cold ingredients to room temperature this typically takes about 30-60 minutes before you begin mixing.

2-Second Step: In a medium mixing bowl, sift together the flour, baking powder, and salt. Sifting incorporates air into the dry ingredients and breaks up any clumps, which helps create a uniform texture in the finished cake. Set this dry mixture aside while you prepare the wet ingredients.

3-Third Step: In the bowl of a stand mixer fitted with the paddle attachment, beat together the granulated sugar, softened butter, and vegetable oil on medium speed until the mixture is smooth and creamy. This process typically takes 2-3 minutes. The combination of butter and oil gives this confetti cake its characteristic rich yet moist texture.

4-Fourth Step: Add the room temperature egg whites in three separate additions, mixing thoroughly after each addition. This gradual incorporation helps the batter maintain its structure. Once all the egg whites have been added and the mixture is smooth, beat in the vanilla extract until the mixture becomes light and fluffy. The egg whites help maintain the pure white color of the cake layers.

5-Fifth Step: With the mixer on low speed, add the flour mixture and buttermilk alternately in three additions each. Begin and end with the flour mixture. Mix gently until just combined, being careful not to overmix which can develop gluten and create a tough cake. The batter should be smooth and slightly thick.

6-Sixth Step: Gently fold in the rainbow jimmies using a rubber spatula. This step should be done by hand rather than with the mixer to prevent breaking the sprinkles and creating streaks of color. Fold just until the sprinkles are evenly distributed throughout the batter.

7-Seventh Step: Divide the batter evenly among the three prepared cake pans. Gently tap each pan on the counter a few times to release any air bubbles and help the batter settle evenly. This simple step helps create more uniform layers with fewer air pockets.

8-Eighth Step: Bake the cakes at 350°F for 32 to 38 minutes, rotating the pans once halfway through baking time. The cakes are done when a toothpick inserted into the center comes out clean or with just a few moist crumbs. Avoid opening the oven door frequently, as this can cause the cakes to fall.

9-Ninth Step: Let the cakes cool in the pans for 10 minutes before removing them. This resting period allows the structure to set slightly, making them less likely to break when removed. Then carefully turn them out onto wire cooling racks to cool completely. The layers must be completely cool before frosting, or the buttercream will melt and slide off.

10-Tenth Step: While the cakes cool, prepare the frosting. Beat the unsalted butter and salt in the stand mixer until soft and smooth. This should take about 2-3 minutes on medium speed. Using softened butter is crucial for achieving a silky smooth texture.

11-Eleventh Step: With the mixer on low speed, gradually add the powdered sugar. Adding it slowly prevents creating a cloud of sugar dust. Once all the sugar has been incorporated, add the heavy cream one tablespoon at a time until you reach your desired consistency. The cream makes the frosting richer and more spreadable.

12-Twelfth Step: Beat in the vanilla extract, then increase the mixer speed to medium and continue mixing until the frosting is fluffy and light. This usually takes another 2-3 minutes. The properly whipped frosting should hold its shape when piped but still spread smoothly over the cake layers.

13-Thirteenth Step: Once the cakes have cooled completely, place one cake layer on a serving plate or cake stand. Spread approximately 1 cup of frosting evenly over the top of this layer, extending just slightly over the edges. This filling helps hold the layers together.

14-Fourteenth Step: Repeat this process with the remaining two cake layers, stacking them evenly on top of each other. If needed, you can use a ruler or cake board to ensure your layers are aligned straight.

15-Fifteenth Step: Set aside approximately 1 cup of the remaining frosting for piping decorative details later. Use the remaining frosting to cover the entire cake with a thin crumb coat first this seals in any loose crumbs. Chill the cake for 15-20 minutes to set this layer, then apply a final, thicker coat of frosting to achieve a smooth finish.

16-Sixteenth Step: Press rainbow sprinkles around the bottom sides of the cake. To do this neatly, place a handful of sprinkles in your palm and gently press them against the bottom third of the cake, working in small sections. You can also use a piece of parchment paper held against the cake to catch excess sprinkles.

17-Final Step: Pipe dollops of the reserved frosting around the top edge of the cake using a piping bag fitted with a large star tip. Garnish the top with additional sprinkles for a festive finish. Your funfetti cake is now ready to serve!