Layer the banana bread batter with swirls of melted Nutella in this order: 1/3 banana bread batter, ¼ cup Nutella spread, 1/3 banana bread batter, ¼ cup Nutella spread, final 1/3 of the banana bread batter with the final ¼ cup Nutella spread swirled on top. Make sure to swirl deep swirls of the Nutella in each banana bread layer.