Preheat oven to 350°F and grease a 9x13-inch baking dish.
In a medium bowl, whisk together pumpkin puree, sweetened condensed milk, pumpkin pie spice, salt, and vanilla extract.
Spread the pumpkin mixture evenly across the bottom of the prepared baking dish.
Sprinkle the dry cake mix evenly over the pumpkin layer without mixing.
Pour melted butter evenly over the cake mix. Sprinkle with chopped nuts if desired.
Bake for 45-55 minutes until the top is golden brown and a toothpick inserted in the center comes out mostly clean.
Allow to cool for 15-20 minutes before serving.
Serve warm with whipped cream or vanilla ice cream.
