Preheat the oven to 350 degrees Fahrenheit. Line three 8-inch round baking pans with parchment paper. Grease with cooking spray or brush with oil.

In the bowl of a stand mixer fitted with the paddle attachment or with an electric mixer, combine the sugar, vegetable oil, eggs and vanilla extract. Beat on medium speed until the ingredients are well incorporated, about 2 minutes, scraping down the bowl as needed. Add the sour cream and beat until well combined.

In a separate mixing bowl, combine the dry ingredients.

Stir in half of the dry ingredients and half the milk into the wet mixture. Beat on low speed until just combined. Add the remaining dry ingredients and milk and mix on low speed until all of the ingredients are well combined. Use a spatula to scrape down the sides of the bowl as needed.

Divide the batter evenly between three bowls. Tint one of the batters with about 8 drops of red food gel. Stir well to combine until the color is fully incorporated.

Tint another bowl of batter with about 8 drops of royal blue food gel color. Stir well to combine. Keep the third bowl of batter as is.

Add small spoonfuls of each color to each prepared baking pan. Scatter colors around. When all batter has been added to the baking pans, using a knife, gently swirl the colors together. Make sure not to over mix them.

Bake for 20 to 23 minutes or until a toothpick inserted in the middle of the cakes comes out clean. When done, remove from the oven and allow to cool completely before frosting.

In the bowl of a stand mixer fitted with the paddle attachment, whip the butter until creamy and light in color, about 5 minutes. Add about 2 cups of powdered sugar, the heavy cream and vanilla. Gradually add the remaining powdered sugar. Once all the powdered sugar has been added, increase the speed to medium-high and beat until light and smooth.

To assemble the cake, use a large serrated knife or cake leveler to remove the domes from the top of the cakes, if needed.

Place the first layer of cake (blue or red) on a serving plate or a cardboard cake round

Place some of the frosting in a piping bag fitted with a large open round tip. Pipe a layer of frosting on top of the cake layer and spread evenly.

Add the second layer of cake (the cake without any food color) and pipe a layer of frosting on top of the cake. Spread the frosting evenly.
Next, add the final layer of cake on top.
Use your piping bag to fill in any gaps between the layers, then frost the outside of the cake. Use an offset spatula or icing scraper to wipe away any excess frosting and smooth the surface of the cake. Use the remaining frosting, to decorate the cake. You can pipe swirls, rosettes or stars if you wish. Garnish with sprinkles - and berries if using. Refrigerate for an hour to allow the cake to set before cutting.