Preheat the oven to 450 degrees F. Grease a large baking sheet with nonstick cooking spray and set aside.
With a sharp knife, cut the potatoes into thick slices, about 1 1/2-inch thick.
Bring a large pot of water to boil. Gently place the sweet potatoes in the boiling water and cook until fork tender, about 15 to 20 minutes. Drain well.
Place the potatoes on the prepared baking sheet and use a fork to lightly smash each piece. Don’t press too hard or they will break apart. You can also use a bottom of a glass and press down. Try to keep them in one piece.
Drizzle the olive oil evenly over the smashed potatoes. Place the sheet in the oven and roast for 25 to 30 minutes, or until golden and crispy.
While the potatoes are roasting, brown the butter. In a small skillet, melt the butter over medium heat, stirring often, until it foams, smells nutty, and browns. This will take about 5 minutes. Scrape the browned butter into a small bowl. Add the maple syrup and cinnamon. Stir until combined.
When the potatoes are crispy, remove the pan from the oven. Turn the oven to broil.
Use a spoon to drizzle the brown butter mixture evenly over the smashed potatoes. Sprinkle the pecans on top and sprinkle with a little flaky sea salt. Place the mini marshmallows on top and place the baking sheet back in the oven. Broil just until the marshmallows are browned, about 1 to 2 minutes. Don’t walk away, watch them closely.
Remove from the oven and serve warm.
Look for long, medium size sweet potatoes, not large fat ones. It will make them easier to smash. I don’t peel the potatoes because the skin makes them extra crispy.