Line a 12-cup muffin pan with cupcake liners (this recipe makes 9 muffins). Add about 1 tbsp of nutella to each cupcake liner. Place in the freezer while you’re making the muffin batter.
Mash bananas in a large mixing bowl, then add eggs, brown sugar, sugar, vegetable oil, sour cream, and vanilla extract. Use an electric hand mixer to mix everything together until smooth, about 3 minutes (it’s ok if a few little chunks remain).
Whisk together the dry ingredients in a separate bowl – all purpose flour, baking soda, cinnamon, nutmeg, and kosher salt. Add to the wet ingredients and continue to mix until fully incorporated, about 30 seconds to 1 minute.
Cover the batter with plastic wrap and allow it to sit for 1 hour at room temperature. At the 30 minute mark, preheat the oven to 375 degrees F.
Once the hour is up, remove the muffin pans from the oven. Take the frozen nutella out of the muffin liner and fill the muffin cup ¾ the way full with the batter. Place the frozen nutella into the batter and fill the muffin cup with batter until just full. Dollop about a teaspoon of nutella on top and use a toothpick to gently swirl the nutella around.
Repeat for the remaining muffin batter. Bake one pan at a time for 15-20 minutes, until a toothpick comes out clean or with just a few crumbs. Allow the muffins to cool in the pan for at least 10 minutes prior to removing.