Preheat oven to 350° and grease a 12-count muffin tin or line with cupcake liners and spray well.
In a medium bowl mix the flour, brown sugar, granulated sugar, baking soda, baking powder, and salt together.
2 cups all-purpose flour, ½ cup brown sugar, ½ cup granulated sugar, 1 teaspoon baking soda, ½ teaspoon baking powder, 1 teaspoon kosher salt
In a large bowl, mix the bananas, sour cream, eggs, vanilla and butter. Gradually add the flour mixture, making sure not to overmix.
3 overripe bananas, ¾ cup sour cream, 2 large eggs, 2 teaspoons pure vanilla extract, 1 cup unsalted butter
Chop chocolate bar into small chunks. Fold walnuts and chocolate chunks into batter.
¾ cup chocolate chunks, ¾ cup chopped walnuts
Fill each cupcake liner nearly full. Add a couple dried banana pieces and chocolate chunks to the top for decorations.
¼ cup dried banana slices
Bake for 20 minutes, or until a knife inserted into the middle comes out clean.
Warm the Nutella in a bowl and drizzle over muffins.
¼ cup Nutella