Preheat oven to 350°F.
Grease a 9x13 baking dish.
In a large mixing bowl, whisk together the pumpkin, sugar, pumpkin pie spice, eggs, evaporated milk, and vanilla until smooth. Pour the pumpkin mixture into the greased pan and spread it evenly.
2 (15 ounce) cans pure pumpkin puree, 12 oz evaporated milk, 1 Tablespoon pumpkin pie spice, 1 cup sugar, 4 eggs, 1 tsp pure vanilla extract
Sprinkle the dry cake mix evenly over the pumpkin mixture. Do not stir.
1 package yellow cake mix
Top with the chopped pecans.
1 cup chopped pecans
Slice the butter into thin pats and arrange them evenly over the top, covering as much of the dry cake mix as you can (or drizzle melted butter over the whole surface).
1 cup butter
Bake at 350°F for 1 hour, until the top is golden brown and a toothpick inserted into the center comes out clean. The center may still be slightly jiggly; it sets as it cools.
Let the cake rest for at least 30 minutes so the pumpkin layer sets.
Serve warm with ice cream or whipped cream.