Preheat oven to 350°F. Line a mini cupcake pan with mini paper liners.
Combine the flour, baking powder, and salt in a small bowl and set aside
1 1/3 cups all purpose flour, 1 1/2 teaspoons baking powder, 1/2 teaspoon salt
In the bowl of an electric stand mixer fitted with a paddle, add the butter and sugar and beat until light and fluffy, about 3 to 5 minutes.
4 tablespoons unsalted butter, 1 cup granulated sugar, 1/4 cup vegetable oil
Mix the vanilla until well blended.
1 1/2 teaspoons vanilla extract
Gradually alternate adding the flour mixture and milk until everything is well combined.
1/2 cup whole milk
In a second bowl beat the egg whites on high speed until stiff peaks form.
3 egg whites
Fold the whipped egg whites and rainbow sprinkles into the cupcake batter.
1/4 to 1/2 cup rainbow sprinkles
Fill each cupcake well about 1/2 to 2/3rds full.
Bake for 10-12 minutes, or until a toothpick inserted into the center comes out clean. Remove from the pan and allow to cool.