In a bowl of a stand mixer, beat the bananas, coconut oil, sugar, yogurt, vinegar, and vanilla on high speed until relatively smooth and creamy, about 2 minutes.
With the mixer still on medium, sprinkle in the baking soda, baking powder, and salt.
Continue with the mixer on medium: add each of the flours. Scrape the sides if necessary, and beat until smooth.
Add in dark chocolate and hazelnuts if using and mix gently to incorporate.
In a prepared muffin tin, evenly distribute the batter amongst all 12 cups.
Bake in the oven for about 20 minutes until the tops are just set/firm.