In a large pot of boiling salted water add the sweet potatoes and cook for 15 to 18 minutes or until tender.
Drain the potatoes and them back to the pot or in a bowl along with 4 tablespoons of butter, ¼ cup of brown sugar, granulated sugar, eggs, salt, nutmeg and cinnamon and mash with a hand masher until smooth. Transfer the mixture to a 13x9 casserole dish, spread out until even and set aside.
Next, add the remaining 4 tablespoons of butter, ½ cup of brown sugar, flour and pecans to a food processor and process on low speed until the pecans are the size of rice.
Add the pecan mixture to the top of the mashed sweet potatoes in the casserole dish and then top off with the marshmallows.
Bake in the oven at 375° for 35 to 30 minutes or until the top is browned.