Spray bottom and line 2 9" cake pans with parchment paper and spray sides.
In large bowl beat your butter and sugar together with hand mixer (or you could use a stand mixer) until light and fluffy.
Add your egg whites and vanilla and continue to mix for a few more minutes until incorporated well.
In separate medium bowl combine your flour and baking powder until combined.
Taking turns add about 1/3 of your flour mixture to your wet mixture, followed by your milk. Continue until all ingredients are added making sure to beat well after each addition.
Divide the batter evenly among your 2 pans.
Place in oven on center rack and bake for about 30-35 minutes or until golden and toothpick inserted into center comes out clean. Remove from oven and let cool in pan for about 5 minutes then turn onto wire cooling rack.
Meanwhile to make buttercream in large bowl beat together your butter and shortening until light and fluffy. Add your almond extract and continue to beat.
One cup at a time add your powdered sugar until thick.
Lastly add in your marshmallow creme and beat until everything comes together.
To make your colored layers in 3 small bowls add about 1/2 cup of buttercream to each and desired amount of food coloring to each bowl.
To assemble and frost your cake, using a cake leveler or serated knife cut each of your cake layers in half. Add one layer to your cake stand, top with your blue buttercream and spread out until about 1/2" from sides. Repeat with remaining layers.
To frost the outside of your cake add your 1/2 cup Marshmallow Fruity Pebbles to buttercream and stir to mix together, add to the sides of your cake and spread around, once spread onto sides and a thin layer on top.
Sprinkle your remaining Marshmallow Fruity Pebbles over top of cake and up sides, gently pushing into frosting.
Serve and enjoy!
Notes
We like to freeze or refrigerate our cakes before serving, it makes cutting the cakes easier and cleaner. It also sets up the frosting.