1½ cups butter, softened and cut into tablespoon-sized pieces
Heat oven to 350F. Grease and flour two 6-inch round cake pans. Set aside.
In the bowl of an electric mixer, beat together the sugar, banana and butter until smooth and well combined, about 2 minutes. Add in the vanilla and eggs, one at a time, beating until combined after each addition.
In a medium bowl, combine the flour, baking powder and salt. Add some of the flour, followed by all of the milk, finishing with the remaining flour mixture. Beat until just combined, scraping down the sides of the bowl as necessary.
Bake 28 to 35 minutes or until a toothpick inserted in the center comes out clean. Cool in pans 10 minutes. Run a knife around the edges of the cake and remove from pans. Cool completely on wire cooling racks, about 1 to 2 hours.
Cut rounded tops off cakes with a serrated knife. Cut each layer in half horizontally to make four total cake layers. Refrigerate cake layers 45 minutes for easier handling.
Meanwhile, make the buttercream:
In small saucepan, combine the egg whites and sugar. Cook, stirring frequently, over medium heat until sugar is dissolved and mixture reaches 160F (food safe temperature) with a candy thermometer. If you don't have a candy thermometer, the egg white mixture should be very warm to the touch and sugar completely dissolved.
Transfer egg white mixture to an electric mixer fitted with a whisk attachment. Beat egg whites on high speed about 8 to 10 minutes or until bowl of electric mixer is no longer warm to the touch. Add the vanilla and the butter, one pat at a time, until the mixture is smooth, creamy and has the texture of buttercream frosting. If the mixture separates while adding the butter, don't worry. Just keep on whisking on high speed until it comes back together.
Place one cake layer on cardboard cake round. Top with some of the buttercream. Repeat with remaining cake layers until you have a four tiered cake. Use a small amount of buttercream to coat entire cake with a crumb coat, about ½ cup buttercream total, just enough to create a smooth surface on the outside of the cake.
Reserve ⅔ cup of buttercream for decorative detailing. Use remaining buttercream to coat outside of cake with a smooth layer of buttercream.
Color reserved buttercream your desired shade of blue, pink or other color. Place in a pastry bag fitted with a ½-inch tube tip. Pipe a dollop of frosting on the top edge of cake. Use the back of a teaspoon to pull the frosting into a petal shape. Repeat with dollops and pulls around the entire top edge of cake.
Use a small round pastry tip to make small dots around the bottom of the cake where the cake meets the cardboard cake round.
Make Ahead Tip: This cake can be made ahead and frosted up to 3 days in advance. Simply make and frost as directed, then store in the freezer. I recommend freezing the cake uncovered for 3 hours first, then lightly covering with plastic wrap and storing in a plastic cake container/carrier in the freezer until ready to serve. To de-thaw, remove from freezer 12 hours in advance and allow cake and buttercream to come to room temperature.
Recipe by Espresso and Cream at http://espressoandcream.com/2014/04/first-birthday-smash-cake-and-diy-bunting.html