Pour cake mix into a bowl; remove and reserve 1 cup. Add 1/2 cup melted butter and 1 beaten egg to the bowl; stir to combine. Press mixture into the bottom of a 9x13-inch baking pan.
Combine pumpkin, evaporated milk, brown sugar, 3 beaten eggs, pumpkin pie spice, vanilla extract, and cinnamon in a bowl. Pour filling into the crust-lined baking pan.
Crumble together the reserved 1 cup dry cake mix, white sugar, and 1/3 cup butter in another bowl using a pastry cutter until crumbly. Sprinkle over the filling.
Bake on the center rack of the preheated oven until top is light brown and the dessert barely jiggles when slightly shaken, 50 to 60 minutes.
Allow to cool completely, about 45 minutes. Beat heavy cream and 2 tablespoons sugar together in a bowl until soft peaks form. Top dessert with whipped cream before serving.