Preheat your oven to 350ºF (175ºC) and prepare your muffin pan by greasing it or lining it with muffin liners.

Mash the ripe bananas in a large mixing bowl, then mix in melted butter, eggs, Greek yogurt, and vanilla extract until creamy.

In another bowl, whisk together the all-purpose flour, baking soda, salt, and sugar.

Gently fold the dry ingredients into the wet banana mixture using a spatula until just combined.

Evenly distribute the batter into your prepared muffin cups, filling them about three-quarters full.

Warm the Nutella in the microwave for about 20 seconds, then swirl a spoonful over each muffin cup and gently mix it into the batter.

Bake giant muffins for 20-25 minutes or regular muffins for 15-20 minutes until a toothpick inserted comes out clean.

Cool the muffins in the pan for about 15 minutes, then transfer them to a cooling rack to cool completely.