Preheat oven to 350°F. Grease a 9x13-inch baking pan.
Whisk together in a large bowl the pumpkin, evaporated milk, eggs, brown sugar, vanilla, pumpkin pie spice, and salt. Mix until completely incorporated and smooth.
Pour the pumpkin mixture into the prepared 9x13 baking dish.
With your hands, sprinkle the dry cake mix evenly over the pumpkin. DO NOT STIR! Try to get the dry cake mix as even and level as possible.
Melt 1 cup of butter, set aside.
In a small bowl, mix 1 cup of chopped pecans, 1 tablespoon melted butter (from the butter you just melted), and 2 tablespoons of brown sugar. Stir together until the pecans are evenly coated.
Pour remaining butter over the top of the dry cake batter. Without mixing it into the cake batter, try to spread it out evenly.
Evenly distribute the sugar-coated pecans on top of the butter.
Bake for 40 minutes, then check the cake; if the top is browning, cover with tin foil and return to the oven and finish baking for an additional 10 minutes (for a total of 50 minutes). Insert a knife; if it comes out clean, the cake is done; if not, bake for an additional 5 minutes.
Allow cake to cool on a wire cooling rack. Once the cake is cooled, place it in the refrigerator to chill and to finish setting up for a few hours before serving.
Top with whip cream and additional chopped pecans (optional) and serve.