Heat oven to 350℉/177℃.
Spray a medium-size casserole with cooking spray. Set aside.
Boil sweet potato chunks for 40 minutes, or until tender. Drain and set aside.
Return pot to the stove.
Over a low heat, melt the butter.
Using a ricer, mash the sweet potatoes into the pot. Mix together.
Add sugar, bourbon, vanilla, nutmeg, and salt. Mix together.
Spoon sweet potatoes into prepared casserole.
Bake for 25 minutes.
Place marshmallows on top.
Bake until marshmallows are lightly browned, about 5 minutes.
Serve.