Preheat the oven to 350°F and line a muffin tin with cupcake liners. In a large bowl, beat together the butter, brown sugar, and granulated sugar until light and creamy. Add the eggs one at a time along with the vanilla extract, mixing until fully combined. Stir in the mashed bananas until evenly incorporated. In a separate bowl, whisk together the flour, baking soda, and salt. Gradually add half of the dry ingredients to the wet mixture, followed by the buttermilk, then add the remaining dry ingredients. Mix gently until just combined—avoid overmixing. Divide the batter evenly among the cupcake liners, filling each about three-quarters full. Bake for 15 to 20 minutes or until a toothpick inserted into the center comes out clean. Allow the cupcakes to cool completely before frosting. For the Nutella frosting, beat together the butter and Nutella until smooth. Add the vanilla extract and 2 cups of powdered sugar, mixing well. Add 1 tablespoon of milk, then gradually add more powdered sugar and milk as needed until you achieve the desired frosting consistency. Spread or pipe the frosting onto the cooled cupcakes.