Preheat the oven to 350°F (175°C). Lightly coat a 9×13-inch baking dish with baking spray. Set aside.
In a large mixing bowl, combine the pumpkin puree, eggs, granulated sugar, brown sugar, evaporated milk, cinnamon, and pumpkin pie spice. Whisk until the mixture is smooth and well blended.
Pour the pumpkin mixture into the prepared baking dish and spread it into an even layer.
In a separate bowl, pour the melted butter over the dry spice cake mix. Use a fork to toss them together until small clumps begin to form. The texture should resemble coarse crumbs.
Sprinkle the buttered cake mix evenly over the pumpkin mixture in the pan, covering the surface completely.
Bake for 60 minutes, or until the top is golden brown and the center is mostly set.
Allow the cake to come to room temperature, then transfer it to the refrigerator. Chill for at least 4 hours or overnight.
Slice and serve cold with a generous dollop of whipped cream.