Preheat oven to 425°. Peel sweet potatoes and cut into 1 inch pieces. Place on a baking sheet and toss with olive oil, 1 teaspoon of minced rosemary and salt. Bake for 30 minutes.
Remove the sweet potatoes and transfer to a large bowl. Turn oven down to 350°F.
Mash the sweet potatoes with a hand masher or fork until almost smooth, leaving it a little chunky. Add the light brown sugar, cinnamon and half and half. Gently stir to combine the ingredients. Spread evenly into an 8" x 8" baking dish.
In a medium bowl, combine the remaining 1 teaspoon minced rosemary, butter cubes, panko bread crumbs, graham crackers and light brown sugar. Using a pastry cutter or fork, cut the butter into the mixture until is well incorporated.
Top the sweet potatoes with marshmallows and crumb topping and bake for 30 minutes or until the topping is golden brown.