Preheat your oven to 350°F (175°C). Grease and flour three 9-inch round cake pans.

In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

In a large mixing bowl, cream the softened butter and sugar together until light and fluffy, about 3-5 minutes.

Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

Gradually add the dry ingredients to the wet mixture, alternating with the milk, and mix until just combined.

Divide the batter evenly into three bowls. Color one bowl with blue food coloring, one with green, and leave one plain.

Spoon the colored batters into the prepared cake pans, alternating colors and creating a marbled effect.

Bake in the preheated oven for 25-30 minutes or until a toothpick inserted in the center comes out clean.

Allow the cakes to cool in the pans for 10 minutes, then transfer to wire racks to cool completely.

Once cooled, level the tops of the cakes if necessary. Place the blue layer on a serving plate and spread a layer of buttercream frosting on top.

Add the green layer, top with more frosting, and finish with the plain layer. Cover the entire cake with a thin layer of frosting to create a crumb coat. Chill for 30 minutes.

Once chilled, frost the cake with the remaining buttercream, using a spatula to create waves and texture.

Decorate the top with edible glitter, mermaid tail topper, and candy seashells and starfish.