Preheat your oven to 350°F (175°C) and grease your cake pans with butter and a light dusting of flour.
Whisk together the flour, baking powder, and salt in a medium bowl and set aside.
Cream the butter and sugar in a large bowl using an electric mixer until light and fluffy, about 4 minutes.
Add the eggs one at a time, beating well after each, then stir in the vanilla extract.
Gradually add the dry ingredients to the butter mixture, alternating with the milk, beginning and ending with the flour.
Divide the batter evenly between the two pans and bake for 25 to 30 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cakes to cool completely on a wire rack before frosting.
Prepare the buttercream by beating the butter until pale, then slowly adding powdered sugar and heavy cream until smooth.
Frost the entire cake with a thin layer of white buttercream to create a base.
Place small dollops of various pastel-colored frostings around the sides of the cake.
Use an offset palette knife to gently smear and blend the colors horizontally, creating the watercolor effect.
Add a simple rabbit face detail on top using a piping bag with a fine round tip if desired.