Preheat oven to 350°F and line pans with cupcake liners.
Beat butter, brown sugar and sugar together until creamy.
Add eggs and vanilla extract and stir until combined.
Add mashed bananas and stir.
In a separate bowl, combine flour, baking soda and salt. Add about half of the dry ingredients to the wet. Add buttermilk and then add the final dry ingredients. Mix only until combined.
Fill cupcake liners 3/4 full and bake for 15-20 minutes or until an inserted knife comes out clean. Let cool.
Nutella Frosting: Beat butter and Nutella. Add vanilla extract and 2 cups powdered sugar. Add 1 Tbsp. milk and then continue to add powdered sugar an milk to reach your desired consistency.