Line a 12-cup muffin pan with cupcake liners and then line another 12-cup muffin pan with 6 liners (this recipe makes 18 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, melted butter, 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 3/4 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.
Notes
Tips for the BEST Muffins EverUse Room Temperature Ingredients: Allow ingredients like eggs and sour cream to come to room temperature before using. This helps them blend better and creates a smoother batter.Measure Ingredients Accurately: Accurate measurements are crucial in baking. I recommend using a kitchen scale and using metric measurements, however if you don't have a kitchen scale, here's an article for how to measure ingredients properly.Don't Overmix the Batter: Overmixing can lead to dense muffins. Mix the batter just until the ingredients are just combined. It's okay if there are a few lumps.Preheat the Oven: Make sure your oven is fully preheated before putting the muffins in. I use an oven thermometer to ensure that my oven is preheated properly to the correct temperature. This ensures even baking and helps the muffins rise properly.Test for Doneness: Use a toothpick or cake tester to check for doneness. Insert it into the center of a muffin – if it comes out clean or with a few moist crumbs (not wet batter), the muffins are done.