Preparing to bake. Heat the oven to 375 degrees. Grease a 1-pound loaf pan.
Mixing the dry ingredients. Whisk together the almond flour, arrowroot powder, and buckwheat flour in a large bowl. (If you want to really aerate your flour, pulse them together in the food processor before beginning to bake.) Add the baking soda and salt and whisk them all together. Set aside.
Combining the wet ingredients. In another bowl, stir together the maple syrup, eggs, coconut oil, and vanilla. Mash the bananas, add them, and whisk until everything is combined well.
Finishing the batter. Add the dry ingredients to the wet, stirring as you go, a bit at a time. When all the flour has disappeared into the batter, and you can't find any more hiding at the bottom of the bowl, add the hazelnuts and stir.
Baking the banana bread. Pour the banana bread batter into the greased pan. Bake until the banana bread is springy to the touch, the edges are pulling away from the pan, and the top is browned, about 45 to 60 minutes. Cool before slicing.