For the Funfetti Cake

• Preheat oven to 350 and line a 9X9 pan with parchment paper and nonstick spray
• In a stand mixer, cream the sugar, butter, and oil for 5 minutes.
• Add in egg and egg whites, mix to combine.
• Stir the vanilla into the buttermilk. Set aside.
• Whisk together the flour, baking powder, and salt. Set aside.
• Alternate adding the dry ingredients and buttermilk to the butter and egg mixture. Start and end with dry ingredients. Stop mixing when a few flour streaks remain.
• Toss the sprinkles with 2 TBSP flour to coat. Then fold sprinkles into batter.
• Pour batter into prepared pan and bake at 350 for 35-40 minutes, or until a toothpick comes out mostly clean with a few moist crumbs attached. Let cool.

For the SMBC

• Add the egg whites, sugar, and pinch of salt to a double boiler over medium heat. 
• Whisk constantly until the sugar dissolves and mixture reaches 150-155 degrees. 
• Pour mixture into stand mixer with the whisk attachment, whisk on high until mixture cools and stiff glossy peaks form, about 5-7 minutes.
• Add vanilla and cake batter flavor. Mix.
• Switch to paddle attachment. Cut butter into chunks. 
• Add butter slowly with mixer on medium speed. 
• Once butter is incorporated, continue to mix until buttercream is smooth. If it curdles slightly, continue to mix and it will come together. 
• Add food color gel if desired.