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.