Heat oven to 350°F / 175°C. Grease a 9x13-inch baking dish.
Whisk pumpkin, evaporated milk, eggs, sugar, spices and salt until completely smooth. Pour into the dish.
Sprinkle the DRY cake mix evenly over the top, covering all the pumpkin. Do not stir.
Drizzle melted butter evenly over the cake mix (cover as much dry mix as you can). Scatter pecans if using.
Bake 50–60 minutes, until edges are set, top is golden, and centre only just jiggles.
Rest 15 minutes. Serve warm with whipped cream. Refrigerate leftovers up to 4 days; reheat slices 20 seconds in microwave.