Heat the oven to 350°F. The oven should be fully hot before baking.
Grease the baking dish lightly so the cake does not stick.
In a bowl, stir together the pumpkin puree, sugar, and eggs. The mixture should look smooth and thick.
Spread the pumpkin mixture evenly in the baking dish. It should form a solid orange layer.
Sprinkle the dry cake mix evenly over the pumpkin layer. Do not mix it in.
Scatter chopped nuts over the top so they are spread out.
Cut the butter into thin slices and place them across the top. Try to cover most of the surface.
Bake uncovered until the top is golden brown and the center looks set, about 50 minutes.
Let it cool slightly before cutting. The cake should hold together but still feel soft.