Make the Cake Layers:

1. Preheat your oven to 350°F (175°C) and grease three 8-inch round cake pans. Line the bottoms with parchment paper.
2. In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
3. In a large bowl, cream the butter and sugar until light and fluffy, about 2-3 minutes. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
4. Alternately add the dry ingredients and the milk/buttermilk mixture to the batter, starting and ending with the dry ingredients. Mix until just combined.
5. Divide the batter evenly among the prepared cake pans and bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
6. Let the cakes cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.

Prepare the Buttercream Frosting:

1. In a large bowl, beat the softened butter until creamy. Gradually add the powdered sugar, 1 cup at a time, mixing well after each addition.
2. Add the heavy cream and vanilla extract, beating until light and fluffy. Adjust the consistency by adding more cream if needed.

Assemble the Cake:

1. Use a serrated knife to level the tops of the cake layers. Set one layer on a cake board or serving plate.
2. Cut a 3-inch hole in the center of the second layer using a round cutter. Place this layer on top of the first layer.
3. Fill the hole with pink or blue candies, ensuring it overflows slightly. Place the final cake layer on top.

Decorate with Ombre Frosting:

1. Divide the buttercream into three bowls. Leave one bowl white, color the second bowl light pink, and the third bowl light blue using gel food coloring.
2. Spread the white frosting around the middle of the cake, the pink frosting on the top third, and the blue frosting on the bottom third. Blend the edges for a smooth ombre effect.
3. Use a star piping tip to pipe rosettes or dollops around the top edge of the cake in alternating pink and blue.