Preheat the oven to 425°F (220°C). Line a large rimmed baking sheet with parchment paper.

Peel the sweet potatoes and cut them crosswise into rounds about 1/2 inch thick. Try to keep the slices similar in thickness so they roast evenly.

Place the sweet potato rounds in a large bowl. Add the 2 tablespoons melted butter, 2 tablespoons maple syrup, 1 teaspoon cinnamon, and 1/4 teaspoon kosher salt. Toss until the slices are evenly coated.

Place the sweet potato rounds in a single layer on the prepared baking sheet, leaving a little space between them. If the slices are crowded, they will steam more than roast.

Roast for 20–25 minutes, turning the rounds carefully about halfway through. They should be tender when pierced with a fork, with lightly browned edges. If the centers are still firm, continue roasting for a few minutes.

While the sweet potatoes roast, combine the 1/2 cup chopped pecans, 1 tablespoon brown sugar, and 1 tablespoon melted butter in a small bowl. Stir until the pecans are evenly coated and crumbly.

Remove the sweet potatoes from the oven: Reduce the oven temperature to 400°F (200°C). Spoon a small amount of the pecan mixture onto each sweet potato round, then top with a few mini marshmallows. Keep the marshmallows mostly centered so they do not slide off as they melt.

Return the baking sheet to the oven and bake for 5–7 minutes, watching closely. The marshmallows should be puffed and lightly golden, while the pecans should look toasted but not dark.

Let the bites stand for about 5 minutes before transferring them to a serving platter. Serve warm while the sweet potatoes are tender and the marshmallow topping is soft.