Preheat oven to 350°F and line 2 cupcake pans with paper liners or with nonstick spray.
In a large bowl, sift together cake flour, baking powder, and salt. Set aside.
In a medium bowl, whisk together buttermilk and egg whites. Set aside.
In a large mixer bowl, combine sugar and butter. Beat for 3 minutes, until well combined and fluffy. Add the vanilla extract and mix until combined.
Alternate adding 1/3 of flour mixture and 1/2 of buttermilk mixture to the mixing bowl, beating until just incorporated in between, be sure to begin and end with the flour mixture. Beat for 1 more minute.
Add sprinkles and stir until just combined.
Fill cupcake liners 3/4 full. Bake for 18-20 minutes, until a toothpick inserted in the middle comes out clean. Transfer to a cooling rack and cool for at least 30 minutes before frosting.
In a large bowl, beat butter until smooth. Add powdered sugar and beat until just incorporated. Add heavy cream and vanilla and mix until incorporated. Increase speed to high and beat for around 4-5 minutes, until frosting is light and fluffy. Add in the sprinkles or jimmies and mix until just incorporated. Pipe or spread onto cooled cupcakes. Garnish with more sprinkles or jimmies.
Don't have buttermilk in the fridge? No problem. Check out my buttermilk recipe here!