Pumpkin Pie Cake

This is one of Dad's favorite Fall treats.
From: Ann Reed
1 pkg. yellow cake mix (reserve one cup)
1/2 c. melted butter
1 egg
Mix the above and press into a 9 x 13 pan. Pat out.
Filling
2 c. canned pumpkin
¾ c. brown sugar
2 ½ tsp pumpkin pie spice
2 beaten eggs
2/3 c. evaporated milk
Mix together until blended and pour over cake.
Topping 1 c. reserved cake mix
¼ c. melted butter
Flatten into glob and lay on filling.
Bake at 350°F for 45 minutes. Test with knife.
Serve with whipped cream.