Heat oven to 350 and place rack in center. Lighlty oil a large ovenproof casserole dish.
For topping, in a medium bowl, stir together brown sugar, nuts, flour and 1/4 cup butter with a fork; set aside.
In a large bowl, stir together potatoes, sugar, coconut, raisins, eggs, vanilla extract and remaining 1/4 cup butter. Stir in cream, combining well. Spoon into prepared casserole dish. Spread topping over sweet potato mixture and bake until golden brown, 20 to 30 minutes.