Preheat the oven to 180°C / 350°F. 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 on top of the crostini. Skewer one marshmallow onto each of six skewers. Roast the marshmallows directly over a campfire until toasted to desired doneness. Push one roasted marshmallow on top of 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 two teaspoons of the chocolate-hazelnut spread over each crostini before topping it with the roasted marshmallows.