Pumpkin Pie Cake

1 (15 or 16 ounce) can pumpkin (not pumpkin pie filling)
1 (12 ounce) can Carnation evaporated milk
1 cup brown sugar, firmly packed
3 large eggs
4 teaspoons pumpkin pie spice
1 (18.25 ounce) box yellow cake mix
1 cup chopped nuts (walnuts or pecans, or mixture)
1 cup melted margarine or butter

Mix pumpkin, Carnation milk, sugar, eggs, and spice together. Pour into a greased 13 x 9-inch cake pan. Sprinkle dry cake mix on top of the pumpkin mixture, then sprinkle nuts on top of cake mix, then pour melted butter evenly over the whole thing. Bake at 350 degrees F for 50 minutes. Refrigerate after it cools. Serve in squares with whipped cream on top.