Preheat the oven to 350°F (175°C) and prepare three 6-inch cake pans.

Mix boxed cake mix, buttermilk, canola oil, sour cream, and eggs for 2 minutes until smooth.

Divide the batter into three bowls and color each with blue, pink, and yellow food coloring.

Pour the colored batters into the prepared pans and bake for 22-27 minutes.

Let cakes cool in pans for 10 minutes, then invert onto a wire rack to cool completely.

Beat unsalted butter until creamy, add powdered sugar, vanilla, and salt. Adjust consistency with milk.

Level off cake layers and layer them with frosting in between and on the outside.

Frost the cake and decorate with rosettes and candy eggs.