Heat the oven to 350°F, and spray a 9x13 inch pan with cooking spray. Set aside.
Combine the pumpkin puree, evaporated milk, brown sugar, eggs, and pumpkin pie spice together in a mixing bowl. Then pour the pumpkin batter into the prepared baking pan.
Spread the cake mix evenly over the pumpkin, then pour the butter over the top.
Sprinkle with pecans and toffee pieces, and place in the oven.
Bake for 50 - 55 minutes, and then check for doneness with a toothpick. Cake is done when it is no longer jiggly in the center. The toothpick may have a little bit of pumpkin residue, similar to a pumpkin pie, on it. This is normal!)
Remove the cake and let it cool for 15 minutes.
Serve warm with whipped cream or ice cream on top.
Notes
Storage:
Fridge: This cake is best served fresh, but leftovers can be covered tightly and stored in the fridge for up to 5 days. I suggest reheating it before serving the leftovers.
Reheat: To reheat, just place the desired portion in an ovenproof dish and bake at 350°F until heated through. You can also microwave it, but the texture may not be as good.