Make the Cake Layers

1. Prepare three 8-inch cake pans with parchment paper circles in the bottom and grease the sides. Preheat oven to 350°F (176°C).
2. Combine the flour, baking powder, baking soda and salt in a medium-sized bowl and set aside.
3. Add the butter, vegetable oil, sugar and vanilla extract to a large mixer bowl and beat together until light in color and fluffy, about 2-3 minutes. Do not skimp on the creaming time.
4. Add the eggs two at a time, mixing until well combined after each. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.
5. Add half of the dry ingredients to the batter and mix until combined. Add the buttermilk and mix until well combined. Add the remaining dry ingredients and mix until well combined and smooth. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated. Do not over mix the batter.
6. Divide the batter into three separate bowls. If you have a food scale, I like to weigh the batter and then divide it evenly. Set one bowl aside. For the other two bowls, add red gel icing color to one and blue gel icing color to the other. Gently stir to combine each until it reaches the right brightness/color.
7. Drop alternating heaping spoonfuls of each batter into the cake pans and gently shake the pan side to side to spread it evenly. Repeat until you’ve used all the cake batter between the three pans. Try to add the same amount of batter to each pan so that they bake evenly.
8. Bake for 20-25 minutes, or until a toothpick inserted comes out clean.
9. Remove the cakes from the oven and allow to cool for about 2-3 minutes, then transfer to cooling racks to cool completely.

Prepare the Frosting

1. Beat the butter until smooth.
2. Slowly add half of the powdered sugar and mix until smooth.
3. Add the vanilla extract, 4-5 tablespoons of water or cream and salt and mix until smooth.
4. Slowly add the remaining powdered sugar and mix until smooth. Add additional water or cream, as needed to get the right consistency of frosting.

Assemble the Cake

1. Use a large serrated knife to remove the domes from the top of the cakes so that they’re flat. These cakes don’t have a large dome, but I like to make sure they’re completely flat.
2. Place the first cake on a serving plate or a cardboard cake round. Spread about 1 cup of frosting evenly on top of the cake. Add the second layer of cake and another cup of frosting. Top the cake with the remaining layer. Spread about 1 cup of frosting evenly on top of the cake.
3. Remove about one cup of the remaining frosting and color it blue. Set aside. Divide the remaining buttercream in half and leave one half white and color the other half red.
4. To add the colored stripes on the sides of the cake, you can add the frosting with an offset spatula or use a piping bag with a large round piping tip. Add alternating stripes of red and white buttercream in even straight lines around the cake. Run an icing smoother around the outside of the cake to smooth it all out. Try to do it as few times as possible to keep the layers from bleeding into each other. Smooth out the top of the cake again, if needed, and finish off the corners.
5. Use the blue buttercream to pipe a border around the top of the cake. I used Ateco tip 847. Finish it off with some white or silver stars, if desired.