It’s hard to imagine anything better than a warm, sticky glazed doughnut or freshly prepared s'mores, but mashing the two together is pretty much the perfect sweet treat. Inspired by a campfire dessert called a Bullseye, this s'mores doughnut sandwich is a gooey mess in the best way, because having melted marshmallow drip down your arm is half the fun. Add a pinch of flaky sea salt if desired to bring a lovely salty-sweet balance to this sugar rush.
Preheat the oven to 400 degrees F or prepare a grill for medium indirect heat. For a gas grill (with 3 or more burners), turn all the burners to medium-high heat; after about 15 minutes turn off one of the middle burners and turn the remaining burners down to medium. For a charcoal grill, bank one chimney starter-full of lit and ashed-over charcoal briquettes to one side of the grill. (Be sure to consult the grill manufacturer's guide for best results.)
Break the chocolate bar into squares. Lightly spray kitchen shears with cooking spray and cut the marshmallows in half crosswise.
Slice each doughnut in half horizontally with a serrated knife. Place the doughnut bottoms, cut-side up, on 2 pieces of nonstick aluminum foil. Top each doughnut bottom with half of the chocolate squares, a pinch of flaky salt if using, 6 marshmallow halves and a doughnut top. Wrap each sandwich in the foil.
Bake or grill over indirect heat, covered, until the chocolate melts and the marshmallows are puffed and gooey, 8 minutes. Let cool slightly before eating!