Preheat the oven to 350 degrees 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 atop the crostini. Skewer
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.