Preheat the oven to 350°F (175°C). Grease a 9×13-inch glass baking dish.
Whisk the pumpkin purée, evaporated milk, brown sugar, eggs, and pumpkin pie spice until evenly combined. Pour the mixture into the prepared dish.
Sprinkle the dry yellow cake mix evenly over the pumpkin filling without stirring. Add the chopped pecans in an even layer.
Pour the melted butter slowly over the surface, covering as much dry cake mix as possible.
Bake for 45–50 minutes. If the center is still liquid, continue in 5-minute intervals, allowing up to about 60 minutes. The cake is ready when the top is golden, the edges are browned, and a thin knife inserted near the center has no liquid pumpkin filling on it.
Cool on a rack for at least 20 minutes before dividing into 20 servings.