Preheat the oven to 350 degrees Fahrenheit and line a muffin tin with muffin liners. This recipe makes 20-24 cupcakes depending how much batter you use, so you can line 2 muffin tins or bake and repeat.
In a small bowl mash the bananas until creamy, it’s ok if there are some lumps. In a medium size bowl combine the 2 cups flour, 1 tsp baking powder, 1/2 tsp baking soda, and 1/2 tsp kosher salt. Whisk until combined.
In the bowl of a hand mixer or stand mixer fitted with the paddle attachment cream the ½ cup butter with the ½ cup granulated sugar, and the ½ cup brown sugar until creamy, about 2 minutes.
Scrape down the bowl and add the egg, 2 egg whites and 2 tsp vanilla extract. Mix for 1 minute on medium speed.
Add in the mashed banana and mix until well combined.
Next add in 1/3rd of the dry ingredients, then half of the buttermilk and alternate, starting and ending with the dry ingredients.
Mix only until combined. Do not overmix.
Using a 3 tbsp cookie scoop, scoop the batter into the muffin liners ¾ of the way full. Bake for 19 minutes or until the cake is set.