Set your oven to 350°F (175°C). Prepare your three 8-inch cake pans by greasing and lining them.
Beat the butter and sugar until it becomes pale. Then, add the eggs one by one and mix in the vanilla.
Mix flour, baking powder, and salt. Add to butter mix alternately with milk, starting and ending with flour.
Separate your batter into six bowls. Color each bowl with a different color.
For thin layers stacked in pans and alternating colors, bake for 18-22 minutes. Use a toothpick to test.
Completely cool off all layers. Level the tops. Center column zone for reveal color, stack with thin buttercream between layers.
Add a core of buttercream in pink or blue before the final crumb coat. Chill it, then complete it with a smooth layer of buttercream.