Topping: Mix brown sugar, milk, flour, butter and pecans together. Spread evenly on top of sweet potato mixture without disturbing mixture.
Casserole: Mash potatoes and combine with sugar, butter, eggs, vanilla, and milk. Place in a greased casserole dish. Spread topping on mixture. Bake at 350 degrees for one hour uncovered.
Variation: During the Christmas holidays, substitute eggnog for the milk in the casserole and topping.
Notes: Blend the sugar and room temperature margarine together first; then add the eggs, milk and vanilla flavoring. I grate a little nutmeg over the casserole. Add the potatoes last. With the topping, I cream the butter with the brown sugar, then add the flour and milk. I add the pecans last.