Make the Funfetti Cupcakes:
- Preheat oven to 350 degrees and line muffin pans with cupcake liners.
- In bowl of stand mixer combine all dry ingredients: flour, sugar, baking powder and salt. Beat for 2 minutes until well combined.
- Add butter a few cubes at a time beating on low until resembles coarse sand.
- Add egg whites and beat on medium speed until well combined.
- Add milk and vanilla and mix on medium speed 1-2 minutes until well combined, scraping sides of bowl as necessary.
- Fold in rainbow jimmies.
- Fill cupcake liners 2/3 full with batter.
- Bake for 18-20 minutes or until toothpick inserted in center comes out clean.
- Let cupcakes cool in pans for 5 minutes, then remove to wire racks to finish cooling.
- Once cupcakes are completely cool frost with Fluffy Vanilla Buttercream.
Make the Fluffy Vanilla Buttercream:
- In bowl of stand mixer beat butter on medium speed for 4 minutes until light and fluffy.
- Add confectioner’s sugar, vanilla and milk and beat on medium speed for 6 minutes, scraping sides of bowl as necessary.
- Use an offset spatula to spread on desired amount of icing.
- Add rainbow jimmies to a bowl and dip the edge of your iced cupcake into the sprinkles and gently twist around in a circular motion until the edges are covered.