Preheat oven to 350°F. Butter a 9×13-inch pan or line with a sheet of parchment, leaving overhang on the long sides for easy removal.
In a large bowl, whisk pumpkin puree, evaporated milk, eggs, granulated sugar, brown sugar, vanilla, pumpkin pie spice, and salt until smooth. Pour into the prepared pan and level the surface.
Evenly sprinkle the DRY yellow cake mix over the pumpkin layer (do not stir). Scatter the chopped walnuts over the cake mix.
Slowly drizzle the melted butter all over the surface, aiming to moisten as much of the dry mix as possible without big dry patches.
Bake for 40 minutes, then check color. If the top is browning quickly, tent loosely with foil and continue baking 10–20 minutes more, until the center looks set and a skewer comes out clean.
Cool the pan completely on a rack, then refrigerate 1–2 hours for the cleanest slices.
Run a butter knife around the inside edges to loosen. Place a large cutting board or platter over the pan, grasp both together, and invert in one motion. Gently lift off the pan and peel away the parchment so the pumpkin is now on top.
Cut into squares and serve with lightly sweetened whipped cream. Store leftovers covered in the refrigerator. Enjoy!