1. Preheat a charcoal or gas grill for direct, medium-high grilling. If using charcoal, push 2/3 of hot coals to 1 side of grill; spread remaining 1/3 on other side. If using a gas grill, reduce heat on 1 side of grill to medium.
2. Dust a 14-inch-wide pizza peel with flour or cornmeal and generously flour a work surface. Divide dough into 4 pieces; roll each to desired thickness. Transfer to peel.
3. Have Nutella within reach of the grill. Slide dough pieces onto hottest part of grill; cover grill. Cook, rotating as needed, until bottoms are blistered and browned, about 2 minutes. Flip dough pieces onto cooler side of grill, cooked side facing up.
4. Cover grill and cook until bottoms are golden, 1 to 2 minutes, occasionally uncovering and checking to ensure they're not burning. If they are, reduce heat (if using a gas grill) or move off grill (if using charcoal). Uncover and spread Nutella on crusts, leaving a 1/2-inch border. Cook until Nutella is melted and glossy. Transfer pizzas to a cutting board and let stand 2 minutes.
5. Cut banana into thin slices on a bias. Arrange banana slices on top of Nutella. Sprinkle with toasted hazelnuts. Top with mint, if desired, and serve.