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

In a large mixing bowl, cream together the butter and sugar until light and fluffy.

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

In another bowl, whisk together the flour, baking powder, and salt.

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

Divide the batter evenly between the prepared cake pans.

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

Allow the cakes to cool in the pans for 10 minutes before transferring to wire racks to cool completely.

In a mixing bowl, beat the softened butter until creamy.

Gradually add the powdered sugar, mixing until smooth.

Add the heavy cream and vanilla extract, and beat until fluffy.

Divide the frosting into three bowls and color each with blue, green, and purple food coloring.

Place one cake layer on a serving plate and spread a layer of frosting on top.

Add the second layer and frost the top and sides of the cake with the colored frosting, creating a swirled effect.

Decorate with edible glitter and place the mermaid tails on top.