Preheat the oven to 350°F. Line a 12-cup muffin pan with paper liners or lightly coat each cup with nonstick cooking spray.
In a large mixing bowl, combine the unbleached all-purpose flour, granulated sugar, baking soda, and table salt. Stir until evenly blended, then set aside.
In a separate medium bowl, combine the mashed bananas, plain yogurt, lightly beaten eggs, melted and cooled butter, and vanilla extract. Stir with a wooden spoon until the mixture is well combined.
Using a rubber spatula, gently fold the banana mixture into the dry ingredients. Mix only until the ingredients are just combined and the batter remains thick and slightly chunky. Do not overmix.
Fold in the toasted, coarsely chopped hazelnuts until evenly distributed throughout the batter.
Divide the batter evenly among the prepared muffin cups.
Place 1 tsp Nutella on top of each muffin. Using a toothpick, gently swirl the Nutella into the top of the batter to create a marbled effect.
Bake for 20 to 24 minutes, or until the muffins are golden and a toothpick inserted into the center of the muffin portion comes out clean.
Remove the muffins from the pan and transfer them to a wire rack. Serve warm or allow them to cool completely before storing.