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Prepare three 8 inch cake pans with parchment paper circles in the bottom and grease the sides. Preheat oven to 350°F (176°C).
Combine the flour, baking powder and salt in a medium sized bowl and set aside.
Add the butter, vegetable oil and sugar to a large mixer bowl and beat together until light in color and fluffy, about 3-4 minutes. Do not skimp on the creaming time. You should be able to see the change in the color and texture of the mixture.
Add the eggs two at a time, mixing until well combined after each. Add the vanilla extract with the second round of eggs. Scrape down the sides and bottom of the bowl as needed to be sure all ingredients are well incorporated.
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.
Divide the batter evenly between 6 bowls. It should be a little more than a cup per bowl.
Add gel icing color to the bowls (one color per bowl) and gently fold until the color is mixed throughout. You should have one bowl each of purple, blue, green, yellow, orange and red.
Add the colored cake batter to the cake pans. Starting with the purple batter, add spoonfuls of batter around the bottom of the pan. Divide each color evenly between the three pans. Give the pan a little side-to-side shake to distribute the batter around a bit between each color. After the purple, add the blue, then green, then yellow, then orange and finally red.
Bake for 28-33 minutes, or until a toothpick inserted comes out clean.
Remove the cakes from the oven and allow to cool for about 2-3 minutes, then remove to cooling racks to cool completely.
To make the frosting, add the butter and shortening to a large mixer bowl and beat until smooth.
Add about half of the powdered sugar and mix until well combined and smooth.
Add the vanilla extract and 2 tablespoons of water or milk and mix until well combined.
Add the remaining powdered sugar and mix until well combined and smooth.
Add additional water or milk as needed to get the right consistency.
To put the cake together, first remove the domes from the top of the cakes so they are level.
Place the first layer of cake on a cake plate or cardboard cake circle, then top it with about 1 cup of frosting and spread into an even layer.
Add the second layer of cake another layer of frosting.
Add the final layer of cake on top, the frost the outside of the cake. Feel free to use my tutorial for frosting a smooth cake, if you like.
Divide the remaining frosting between 6 bowls. I had about 1/3 cup of frosting per bowl, but you may have a little more or less, depending on how much you use to frost the cake.
Use the gel icing colors to color each bowl of frosting, using one color per bowl.
To make the rainbow swirl frosting border, add each color of frosting to a ziplock bag. Lay a piece of clear wrap out on the table. Trim the corner off of each ziplock bag. Pipe a thick row of each color of frosting in this order – purple, blue, green, yellow, orange, red – side-by-side onto the clear wrap.
Fold the clear wrap over into a long log (see photos above for guidance). Twist one end closed and trim the other end, if needed, so that the frosting is right at the end.
Put the log into a piping bag with the open end of the log going into the piping tip. I used piping tip Ateco 849.
Begin piping shells of frosting around the top edge of the cake. After each shell is piped, turn the piping bag just a little bit before piping the next shell. As you continue piping and turning the piping bag, you’ll get a rainbow that rotates around so you see all the colors.
To pipe shells around the bottom of the cake, create another log of rainbow frosting, add it to the piping bag and pipe shells around the bottom in the cake manner – turning the bag slightly after each shell.
Store cake in an airtight container until ready to serve. Cake is best for 2-3 days.