Preheat oven to 350 degrees F. Line a muffin tin with 12 cupcakes liners.
In a large mixing bowl, combine flour, cornstarch, baking powder, baking soda, and salt. Set aside.
Use can electric mixer to beat butter and sugar together until light and fluffy. Mix in the the banana, buttermilk, and eggs until combined. Add in the dry ingredients a little at a time while mixing, until combined. Divide batter among the liners (an ice cream scoop works perfect for this!) and bake for 15-18 minutes or until toothpick comes out clean.
Remove cupcakes from tin and allow to cool on a rack.