Step 1: Preheat your oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners. If you wish to make more muffins as the original recipe mentions, prepare a 6-cup muffin tin with liners as well.

Step 2: In a medium bowl, whisk together the all-purpose flour, baking soda, baking powder, and salt. This ensures that the dry ingredients are evenly distributed, preventing lumps and creating a consistent rise in the muffins.

Step 3: In a large bowl (or the bowl of a stand mixer), mash the ripe bananas until mostly smooth, leaving a few small chunks for texture. Add the melted and cooled butter, brown sugar, granulated sugar, eggs, and vanilla extract. Beat on medium speed for 3 minutes or until well combined and slightly fluffy. This step is crucial for incorporating air into the batter, resulting in lighter, more tender muffins. The slightly longer mixing time helps to break down the bananas further, releasing their natural sweetness and moisture. If you don't have a stand mixer, a hand mixer or a sturdy whisk will work just fine; just be prepared for a little extra elbow grease!

Step 4: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix, as this can lead to tough muffins. A few streaks of flour are okay at this stage. Overmixing develops the gluten in the flour, resulting in a denser, less tender crumb.

Step 5: Warm the Nutella in a microwave-safe bowl for 20-30 seconds, or until it is softened and easily spreadable. This will make it easier to swirl into the muffin batter.

Step 6: Fill each muffin cup about ¾ full with the banana batter. Drop a dollop (about 1 teaspoon) of the warmed Nutella on top of each muffin. Use a toothpick or a thin knife to gently swirl the Nutella into the batter, creating a marbled effect. Don't over swirl, as this can muddy the colors of the muffins and make the look unappealing.

Step 7: Bake for 15-18 minutes, or until a toothpick inserted into the center of a muffin comes out clean. Keep a close eye on them during the last few minutes of baking to prevent them from overbaking.

Step 8: Let the muffins cool in the muffin tin for a few minutes before transferring them to a wire rack to cool completely. This helps prevent them from sticking to the pan and allows them to cool evenly.