Preheat your oven to 350°F (175°C) and grease three 8-inch cake pans with butter or non-stick spray. You can also line the bottom of each pan with parchment paper to make removing the cakes easier.

Whisk together the flour, baking powder, baking soda, and salt in a medium bowl. Set aside.

In a large bowl, cream the softened butter and sugar until light and fluffy, about 3-4 minutes.

Add the eggs one at a time, beating well after each addition.

With the mixer on low speed, add the dry ingredients to the bowl in three parts, alternating with the buttermilk in two parts. Begin and end with the dry ingredients. Be careful not to overmix, as this can make the cake tough.

Stir in the vanilla extract until just combined.

Divide the batter evenly into six small bowls. Add a few drops of food coloring to each bowl, creating red, orange, yellow, green, blue, and purple batters.

Starting with the red batter, pour it into the first prepared pan and spread it evenly. Repeat with the remaining colors, layering them in the order of the rainbow. Tap the pans gently on the counter to remove any air bubbles. Bake for 18-20 minutes or until a toothpick inserted in the center comes clean. Allow the cakes to cool completely in the pans before removing them.

In a large bowl, beat the softened butter until creamy. Gradually add in the powdered sugar, one cup at a time, until the frosting is smooth and fluffy. Stir in the vanilla extract and heavy cream, adding more or less cream as needed to achieve your desired consistency.

Once the cakes have cooled, use a serrated knife to level off any domed tops. Place one cake layer on a serving plate or cake stand and spread a layer of frosting on top. Repeat with the remaining layers, making sure to spread an even layer of frosting between each layer. Once all the layers are stacked, use the remaining frosting to cover the top and sides of the cake. Use a spatula to create smooth edges and swirls.

To decorate the cake, you can use sprinkles, edible glitter, or fresh fruit. Cut the cake into slices and serve immediately, or store in the refrigerator for up to three days.