Line a 9x5 inch loaf pan with parchment paper, smoothing any creases you see and folding the ends over the sides, this is so much easier than spraying a pan! If desired, lightly spray the parchment. Preheat oven to 350° (325° for dark, stoneware* or glass pans)
In a large mixing bowl or bowl of a stand mixer, cream your sugar and butter, if using all natural cane sugar, cream for a minute or two in order to break down the larger sugar grains. Add eggs, one at a time and mix on low speed for 1 minute each. It will have a bit of a curdled look. Scoop in your mashed bananas, vanilla and milk, mixing until well combined.
Using a sieve, place over top of batter and measure your all purpose flour, oat flour, baking soda and salt into the sieve, sifting gently into the bowl. Mix gently until just combined, scraping to incorporate any rogue flour.
In a small microwave safe bowl, measure Nutella and microwave (if needed) for 15-20 seconds. Stir in 1 cup of banana bread batter until combined. Spoon layers of batter into the prepared loaf pan, starting with your banana bread batter, add more banana bread batter first, then a good portion of Nutella batter, then balance of banana bread batter, finishing with the rest of the Nutella batter.
Using a chopstick, a knife, or a small wooden spoon handle, gently swirl the two batters. Don't do so much you combine, you just want to swirl the two around. I found reaching down (with a chopstick) and slowly pulling up some of the batter while pulling the opposite direction worked well.
Bake for 50-70 minutes, check loaf using a toothpick, be sure to insert into the chocolatey portion as well as banana. Using a stoneware pan, I had to bake for 70 minutes at 325°, but started checking on it at 50 minutes. Let cool in pan on a cooling rack for 15 minutes, then lift out of pan using parchment paper to cool completely.
If your loaf sinks in the middle, all is not lost, it'll be soft and maybe a bit gooey in the middle, but toast on a pan under the broiler for a minute, then just slather it with butter, eat with a fork and dream happy, gooey Nutella thoughts! Loaf may be frozen for later use, or freeze half, eat half, give away half!
Notes
If baking at sea level or under 5260 feet your time in the oven will probably be closure to 50-60 minutes.Try using a light, whole wheat flour in place of the all-purpose flour, it may turn out a bit dry, but reduce time slightly in the oven, still tasty when toasted!I personally love my stoneware pan, it tends to heat evenly and brown beautifully, but bakes a bit slower.