Funfetti Cupcakes

Preheat oven to 350 degrees. Line a cupcake pan with 12 liners and set aside.
In a medium bowl, whisk together the cake flour, baking powder, baking soda and salt. Then set aside the dry ingredients.
In a large bowl, cream together the butter and granulated white sugar with an electric hand mixer on high speed until fluffy, about 2-3 minutes. (If using a stand mixer, use the paddle attachment.)
Next add in the egg whites and vanilla and mix on medium-high speed for 1 to 2 minutes until pale and smooth.
Alternate adding the dry ingredients and the buttermilk to the butter mixture a little at a time until each has been added completely, mixing on low speed, then medium speed for each addition. Mix just until the batter is combined and smooth, scraping the sides of the bowl as necessary. The batter will be thick.
Add in the rainbow jimmies and mix on low speed just until combined.
Divide the batter evenly among the 12 liners until each is two-thirds to three-quarters full.
Bake the cupcakes for 16 to 18 minutes or until a cake tester or toothpick comes out clean from the centers. Let the cupcakes cool in the pan for 10 minutes, then transfer them to a cooling rack to finish cooling.

Add the butter and salt to a large bowl. Whip with an electric mixer on high speed for 5 to 10 minutes until the butter is pale, fluffy and double in size.
Pour in the melted white chocolate and combine on medium speed.
Then sift the powdered sugar into the mixture ½ cup (65 g) at a time. Mix on low, then medium speed, making sure each addition is fully combined before adding the next. Scrape the sides of the bowl as necessary.
When the last addition is incorporated, mix on high speed for about 1 minute until the frosting is light and fluffy. Transfer the frosting to a piping bag fitted with a decorative tip.
Once the cupcakes are cool, pipe white chocolate buttercream onto each. Sprinkle extra sprinkles on top of each and serve!