Toast the hazelnuts in a frying pan over medium heat for about 4 to 5 minutes, or until lightly browned and fragrant. Place them in a clean kitchen towel and rub them to remove the skins. When they are cool enough to be handled, chop them coarsely; set aside.
Preheat the oven to 350º F. Line a muffin pan with 12 paper liners, or use silicone muffin molds.
In the bowl of a stand mixer, beat the eggs with the sugar until light and pale in color. Stir in the melted butter and the yogurt. In another bowl, combine the flours, salt, baking powder, and baking soda. Stir the dry ingredients into the egg mixture, until just combined. Add the bananas, hazelnuts, and chopped chocolate, and mix gently.
Divide the batter among the muffin cups, place in the oven, and bake for 25 to 30 minutes, until the blade of a sharp knife inserted in the middle of the muffins comes out almost dry. Remove from the oven and let cool briefly, then invert onto a cooling rack to cool completely.
If you freeze some of the muffins, reheat them for 1 minute in the microwave, or for a few more minutes if you use a toaster oven or an oven preheated to 300ºF (150ºC). They’ll be as if they were coming fresh out of the oven. Think about all of that melting chocolate.