Set oven to 350ºF (175ºC). Apply non-stick cooking spray to the bottom and sides of a 9x13 inch cake pan to prevent sticking and ensure easy removal after baking.
In a large mixing bowl, combine the canned pumpkin puree, evaporated milk, packed brown sugar, eggs, and pumpkin pie spice. Whisk until fully blended and smooth. Pour this pumpkin batter evenly into the prepared cake pan.
Sprinkle the dry yellow cake mix as evenly as possible over the entire surface of the pumpkin batter in the pan. Next, evenly scatter the chopped pecans over the cake mix layer. Finally, slowly pour the melted unsalted butter over the top, making sure to cover as much of the surface as possible. Drizzle the butter slowly to help create a crisp, golden top.
Transfer the pan to the preheated oven and bake for 45-50 minutes. The edges should be golden brown and the center should be set. Test doneness by inserting a knife in the middle; it should come out clean. Once finished, allow the cake to cool slightly before serving. For extra flavor, sometimes dust the cooled cake with a light sprinkle of powdered sugar or serve with whipped cream.