Preheat oven to 350 degrees F. Line muffin tins with cupcake liners (12 regular, 24 mini, or a combo of 6 regular and 12 mini).
In a mixing bowl, combine flour, baking powder, baking soda, and salt.
In another bowl, melt butter and whisk in 1 cup sugar, buttermilk, sour cream, and vanilla. Add the liquid ingredients to the dry and mix until just combined. Set aside.
With an electric mixer, beat egg whites until soft peaks form. Fold into the batter until combined.
Spoon batter into cupcake liners (I like to use an ice cream scoop).
Mix the remaining 1/4 cup sugar and cinnamon together and sprinkle 1/2 teaspoon on the top of batter. Reserve remaining.
Bake for 15-18 minutes until toothpick comes out clean. (If you are baking mini and regular cupcakes, bake the mini ones first and then the regular.) Leave in tin for a couple minutes until able to remove onto rack. Cool completely before frosting.