Preheat the oven to 350ºF. Spray 6 giant or 12 regular muffin cups with cooking spray and set in a muffin tin.
In a large bowl, combine the mashed bananas with the melted butter, stirring to combine. Mix in the eggs, greek yogurt, and vanilla. Set aside. In a separate bowl, combine the flour, baking soda, salt, and sugar. Fold the dry ingredients into the wet.
Pour the batter evenly into the muffin cups. Melt Nutella in microwave for 20 seconds. Drizzle Nutella over muffins, then gently swirl the batter with a knife to form swirls of Nutella. If using giant muffin cups, bake for 20-25 minutes, or until a knife inserted in the center comes out clean. If using small muffin cups, bake for 15-20 minutes, or until a knife inserted in the center comes out clean. Allow to cool for 15 minutes before serving!