Preheat your oven to 350°F (175°C).
Grease a 9x13 inch baking dish.
In a large bowl, combine pumpkin puree, evaporated milk, eggs, sugar, vanilla, and pumpkin pie spice. Mix until smooth.
Pour the pumpkin mixture into the prepared baking dish.
Evenly sprinkle the dry cake mix over the pumpkin layer.
Drizzle the melted butter over the cake mix.
If desired, sprinkle chopped nuts on top.
Bake for 50-60 minutes or until the center is set.
Let cool for at least 15 minutes before serving.
Serve warm with a dollop of whipped cream or vanilla ice cream.