Preheat oven to 350℉. In a large mixing bowl, combine the evaporated milk, pumpkin puree, brown sugar, eggs, and pumpkin spice mix until evenly incorporated.
Spray your casserole dish with oil. Carefully pour in the pumpkin mixture.
Next, sprinkle the box cake mix on top of the pumpkin mixture (do not mix it in). Then, sprinkle with the chopped pecans and top with the melted butter.
Bake for 50 minutes, uncovered.
Remove from the oven and let cool for 15 minutes. The bottom will be a soft, goopy texture, so if you want it firmer you can refrigerate before cutting into it.