1. Divide the chocolate hazelnut spread among the tortillas and spread evenly over half of the surface of each.
2. On a griddle or in a large skillet over medium heat, melt 2 tablespoons of the butter until bubbly. Add 1 or 2 of the tortillas, spread-side up, to the griddle. Spread 1/3 cup marshmallows in an even, single layer over the chocolate spread on each tortilla. Cook until the tortillas bubble in the pan and turn light golden brown on the bottom.
3. Using a spatula, quickly fold the uncovered half of the tortilla over the marshmallows, making a half-circle. Press on the top of the tortillas gently to seal. When the marshmallows are slightly melted and spongy, flip the crêpe and let stand in the pan until the underside is just hot; do not overcook.
4. Transfer the crêpes to a cutting board and let stand for 1 minute. Repeat with the remaining butter, tortillas, and marshmallows.
5. To serve, put the chopped hazelnuts on a plate. Cut the crêpes in half crosswise, and then quickly press the cut side and round edges into the nuts to adhere. Sprinkle additional nuts over the top and serve immediately garnished with banana slices, if desired.