Begin by freezing Nutella into small balls. Place these balls on a baking sheet lined with parchment paper and freeze for at least 30 minutes (if you do this first, they should be ready by the time you've prepped everything else).

Preheat the oven to 425°F.

In a mixing bowl, combine melted butter and sugar. Whisk until well blended. Add the mashed bananas and mix thoroughly. Then add the Greek yogurt and vanilla extract, mixing until combined.

In another bowl, sift together the all-purpose flour, salt, and baking powder. Gradually fold these dry ingredients into the wet mixture until just combined. Gently fold in the chopped chocolate.

Line a muffin tray with 12 paper muffin cups. Fill each cup halfway with the batter. Place a frozen Nutella ball in the center of each cup, then cover with more batter until the cups are almost full. Sprinkle additional chopped chocolate on top.

Bake in the preheated oven at 425°F for 10 minutes, then reduce the heat to 350°F and continue baking for an additional 15 minutes until a toothpick inserted into the muffins comes out clean.

Once baked, remove the muffins from the oven and allow them to cool completely before serving.