Mix together cooked/mashed sweet potatoes, margarine, eggs, vanilla, salt, sugar, flour, and cinnamon and put in a 9 x 13 casserole dish.
In another bowl, mix together topping ingredients: flour, brown sugar, and melted butter. Spread evenly over casserole. Sprinkle walnuts on top (Grandma puts walnuts on one half, leaving the remaining half for non-nut lovers).
Bake @ 350 for 30 minutes.
Grandma says: I find that (baking for 30 minutes) is true if the sweet potatoes are still pretty warm. Otherwise I thinks it takes longer. At least 45 minutes or more 'til good and hot.
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