Preheat your oven to 350°F (175°C) and generously grease three 9-inch round cake pans with cooking spray. Line the bottom of each pan with parchment paper.

In a large mixing bowl, whisk together 5 1/2 cups of all-purpose flour, 1 tablespoon of baking powder, and 1 teaspoon of kosher salt.

In a separate bowl, use an electric mixer to beat together 1 cup of softened unsalted butter and 3 cups of granulated sugar until light and fluffy.

Add 6 large eggs, one at a time, and stir in 1 tablespoon of pure vanilla extract until fully combined.

Carefully alternate adding the dry ingredients and 1 cup each of whole milk and sour cream to the wet mixture.

Divide the batter into bowls, add food coloring to each, and mix until desired colors are achieved.

Pour each colored batter into the prepared cake pans and bake for 25-30 minutes, or until a toothpick comes out clean.

Let the layers cool in the pans for 10 minutes before transferring them to a wire rack.

Once cooled, carefully cut out small sections in the center of each layer and fill with 1 1/2 cups of Lucky Charm marshmallows.

Stack the layers using buttercream frosting to hold them together, then frost the exterior with buttercream.

Decorate with green, black, and yellow food coloring and add extra Lucky Charms on top for a finishing touch.