01 - Preheat the oven to 350°F and line a 12-cup muffin tin with paper liners.
02 - In a large bowl, whisk together the mashed bananas, eggs, oil, yogurt, and vanilla extract until smooth and well incorporated.
03 - In a separate bowl, combine the flour, sugar, baking powder, baking soda, and salt, stirring to ensure even distribution.
04 - Gently fold the dry ingredients into the wet mixture until just combined. Avoid overmixing to maintain tender texture.
05 - Divide the batter evenly among the prepared muffin cups, filling each about two-thirds full to allow for rising.
06 - Warm the Nutella slightly if needed. Drop about 1 teaspoon on top of each muffin and use a skewer or toothpick to swirl into the batter creating a marbled effect.
07 - Bake for 18-22 minutes, or until a toothpick inserted into the center comes out clean except for melted Nutella.
08 - Let the muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.