I love this, and I think I shall forever more make banana bread in this fashion. I started with this recipe from Cooking Light as inspiration. However, I don't know if you want to use that recipe, because while it was low fat, it was bit on the dry side. You can really use any of your own, favorite banana bread recipes.
1 recipe for plain banana bread batter
In a pyrex measuring cup, measure about 1/3 to 1/2 cup of Nutella. Microwave it for about 30 seconds until it's soft. Measure out 1 cup of banana bread batter and mix it with the Nutella. Alternate spooning in the plain batter and the Nutella batter until it is all used. Take a small paring knife and swirl around until the batter looks marbled. Bake as directed.