Grease a 9 x 13" baking dish. Set aside 1 cup of yellow cake mix.
Mix remaining cake mix, 1/2 cup butter and 1 egg well and press in the bottom of baking dish.
Mix pumpkin pie layer ingredients together well and spread evenly over cake layer.
To make the crumbly layer - mix cake mix, sugar and margarine together until crumbly. Sprinkle over filling. Sprinkle pecans on top.
Bake at 350 degrees for about 1 hour 10 minutes or until set. Serve warm with whipped topping.
*check doneness with a toothpick around 50 minutes to account for variations in ovens and avoid burning the cake. The cake is done with the toothpick is clean when inserted in the middle.