Preheat the oven to 350°F. Butter and flour 3 six inch cake pans and set aside (you could also use 8 inch pans instead, but your layers will be thinner).
In a large bowl or the bowl of a stand mixer, whisk together the cake flour, all purpose flour, sugar, baking powder, and salt. Add the butter and mix until the mixture resembles damp sand. Beat the eggs in one at a time. Add the sour cream and vanilla, and mix until smooth.
Separate the batter into three bowls, dividing evenly. Add drops of food coloring to each bowl to create a yellow batter, a pink batter, and a blue batter (I used two drops of yellow, three drops of pink, and six drops of blue). Spoon the batter into the prepared pans and spread evenly.
Bake 25-30 minutes, or until the top springs back when touched lightly and a toothpick inserted in the center comes out clean. Remove and let cool completely.
While the cake cools, make the frosting by adding the butter and marshmallow cream to a large bowl or the bowl of a stand mixer. Beat until smooth. Add the powdered sugar, vanilla, and 2 tablespoons half and half, and beat until smooth. Increase the speed and beat 5-6 minutes, until light and fluffy. Add additional half and half as needed to reach a light consistency.
When the cakes have cooled, level each layer using a cake leveler or serrated knife.
Place the blue layer, cut side up, on a six inch cake round (or a six inch circle of cardboard covered in foil). Spread a layer of frosting over the blue layer, and place the pink layer on top. Spread another layer of frosting over the pink layer, and place the yellow layer on top, cut side down. Spread a thin layer of frosting over the entire cake to seal in the crumbs. Refrigerate 20-30 minutes.
Remove the cake from the refrigerator and spread a thicker layer of frosting over the outside. Sprinkle the top of the cake with toasted coconut, and arrange Cadbury eggs in the center to look like a bird’s nest.