Line a large baking sheet with a silicone baking mat, parchment paper, or foil. Arrange the bread slices on the baking sheet. Generously brush the butter over the bread slices. Sprinkle the slices with sugar. Bake until the crostini are crisp and golden, about 15 minutes.
Arrange the chocolate pieces in a single layer atop the crostini. Skewer 1 marshmallow onto each of 6 skewers. Roast the marshmallows directly over a campfire until toasted to desired doneness. Push 1 roasted marshmallow atop the chocolate on each crostini and serve. Repeat to make 24 s'mores total.
Alternative: Chocolate-hazelnut spread (such as Nutella) may be used instead of the chocolate bars. To do so, omit coating the bread slices with sugar before baking them, and spread about 2 teaspoons of the chocolate-hazelnut spread over each crostini before topping it with the roasted marshmallows.