Make cupcakes.

• Prep oven and pans. Position rack in the center of your oven and preheat the oven to 350° F (180°C). Line a cupcake pan with paper liners. Set aside.
• Combine dry ingredients.  In a medium mixing bowl, combine the cake flour, baking powder and salt. Whisk together and set aside. 1 1/4 cups (150 g) cake flour 1 1/4 teaspoons baking powder 1/2 teaspoon salt
• Cream butter and sugar Cream together the butter and sugar to the bowl of an electric mixer fitted with the paddle attachment. Beat on medium-high speed for 3 minutes, until light and fluffy in color. Scrape down the sides of the bowl. 1/2 cup (1 stick, 113 g) unsalted butter 3/4 cup (150 g) granulated sugar
• Add eggs and vanilla. Mix in the eggs one at a time until incorporated. Scrape down the sides of the bowl after each addition. Add vanilla and beat to combine. 2 large eggs 1 tablespoon pure vanilla extract
• Alternate dry ingredients with buttermilk. With the mixer on low speed, add the dry ingredients alternately with the buttermilk, beginning and ending with the dry ingredients and mixing just until incorporated. Scrape down the sides of the bowl and mix for 15 seconds longer. 1/2 cup (120 ml) buttermilk
• Color the cupcake batter. Divide the cupcake batter between six bowls. Add a few drops of food coloring to each bowl. I used purple, blue, green, orange, yellow and red.
• Divide cupcake batter. Divide the batter between the prepared paper liners. Add a small spoonful of each color to each cupcake well. I started with purple, then blue, green, yellow orange then red.
• Bake cupcakes. Bake 18-20 minutes, or until a toothpick inserted in the center comes out clean. Allow to cool in the pans 5-10 minutes, then transfer to a cooling rack to cool completely before frosting.

Make frosting.

• Mix butter and sugar. In a mixing bowl, beat together the butter and powdered sugar together until light and fluffy, about 2-3 minutes. 1 1/4 cups (2 1/2 sticks, 283 g) unsalted butter 2 1/2 cups (283 g) powdered sugar
• Add fluff. Add the marshmallow fluff to the mixture and beat until combined. Scraping down the bowl as needed. 7 ounces marshmallow fluff
• Add heavy cream, vanilla, and salt. Add heavy cream, vanilla, and salt. Add in your heavy cream, vanilla extract and salt, and mix on medium to high speed for 7-8 minutes until light and creamy. If not using immediately, store in a container in the refrigerator until ready to use, mix again until light and fluffy. 2 Tablespoons heavy cream 2 teaspoons pure vanilla extract Pinch of salt

Frost cupcakes. With a large piping tip fitted into a large piping bag, pipe frosting onto cupcakes. Top with sprinkles if desired!