Grease three 7-inch spring-form pans or (or three 8-inch pans) with butter; set aside.
Remove 1/4 cup of cereal and set aside for later use.
Melt the butter in an extra-large pot over medium heat Add marshmallows; stir until melted. Add vanilla extract and stir to combine. Add remaining box of cereal contents to the marshmallow mixture. Stir together with a large spoon until all the cereal is coated with marshmallow. Quickly divide and press the cereal into the pans with well-buttered hands. I often cover a length of wax paper or parchment paper with cooking spray and press it on top of the cereal mixture to pack it down into the pans (be careful –it’s hot!). Refrigerate cake layers in the pans until well chilled and set, about 2 hours.
For the frosting
Mix together the butter and confectioners’ sugar in a stand mixer fitted with the whisk attachment. Begin mixing on low speed until crumbly, and then increase to high and beat for 3 minutes, adding milk or cream as needed to thin the mixture. Add vanilla and beat again for another minute until light and fluffy.
Transfer 1/3 cup frosting to a piping bag fitted with a large star tip; set aside.
Unmold Fruity Pebbles treat layers from the spring-form pans and frost the tops of two cake layers. Stack layers with the unfrosted layer on top; gently press layers together. Place on a cake stand. Frost the outside of the cake with a thin layer of buttercream. I used just enough to fill in the cracks and crevices and then scraped the excess off with a large offset spatula so the cereal is visible on the cake’s edges. You may use more if desired.
Pipe stars along the top edge of the cake with the reserved frosting in the piping bag. Scatter reserved 1/4 cup of cereal on top of the stars. If birthday candles are being used, pipe more frosting onto the center of the cake and stand candles upright in the frosting. Let cake come to room temperature before cutting. Use a large chef's knife to cut the cake into slices.
Keyword american buttercream, fruity pebbles cereal, mini marshmallows