Funfetti Layer Cake

Ingredients

• 1 1/2 tsp baking powder
• 1/2 tsp salt
• 1/4 tsp baking soda
• 2 1/4 cups all purpose flour
• 1 1/2 cups granulated white sugar
• 1 tbsp vanilla bean paste
• 3/4 cup buttermilk, at room temperature
• 4 egg whites, at room temperature
• 2 1/2 cups powdered sugar, sifted
• 1/2 cup + 2 tbsp unsalted butter, softened
• 2 cups unsalted butter, softened
• 1/2 cup sour cream, at room temperature
• 1/2 cup rainbow jimmies
• 8 oz. white chocolate, melted and slightly cooled
• 1/4 cup rainbow jimmies (for decorating)
• 1/4 cup rainbow nonpareils (for decorating)