Heat the oven: Preheat to 350°F and grease a 9 x 13-inch baking dish with nonstick cooking spray.
Make the filling: In a large bowl, whisk together the pumpkin, brown sugar, eggs, evaporated milk, vanilla, cinnamon, ginger, nutmeg, allspice, salt, and cloves until smooth and completely combined. Pour the mixture into the prepared baking dish.
Add the cake mix: Sprinkle it evenly over the pumpkin filling, breaking up any clumps. Do not stir. Scatter the pecans over the top, if using.
Layer on the butter: Arrange the very thin butter slices evenly over the surface, covering as much of the dry cake mix as possible.
Bake: Bake for 60 to 65 minutes, until the topping is golden brown and the filling is set around the edges with just a slight jiggle in the center. If the pecans brown early, tent loosely with foil.
Let it rest: Cool for at least 30 minutes before serving. The filling keeps setting as it cools. Serve warm, at room temperature, or chilled with your favorite toppings.