Step #2 Butter a 13 x 9-inch or an 11 x 7-inch baking dish.
Step #3 Bake the sweet potatoes in the oven until fork tender; cool, peel & mash with a fork (the potatoes can be boiled with skin on also).
Step #4 In a bowl mix the mashed sweet poatoes with 1/2 c brown sugar, vanilla, eggs, half & half & melted butter; mix with an electric mixer until mixd.
Step #5 Transfer to prepared baking dish.
Step #6 Combine the remaining 1 c brown sugar with the flour.
Step #7 Cut the cold butter in with a pastry knife until crumbly.
Step #8 Toss mixture with sliced pecans.
Step #9 Sprinkle top evenly over the top of the casserole.
Step #10 Bake for about 30 mins, topping with the marshmallows (if using) after about 20 mins of baking.