In a large bowl, combine the pumpkin, milk, eggs, brown sugar, pumpkin pie spice, and vanilla extract. Stir until blended and pour into a 9x13-inch baking dish.
Sprinkle the dry cake mix evenly over the pumpkin. Place the small pieces of butter over the cake mix, as evenly as possible (as they melt they will spread out), and sprinkle pecans over the top of the cake.
Bake for 50 minutes.
If you really enjoy the fall spice flavors, consider using a spiced cake mix instead of the yellow cake mix!