Lightly coat the inside of a 3-1/2- or 4-quart slow cooker with cooking spray. In prepared cooker stir together pumpkin, evaporated milk, sugar, and 1 tablespoon of the pumpkin pie spice. Spread batter evenly in the bottom of prepared cooker.
In a medium bowl stir together cake mix, pecans, and the remaining 1 tablespoon pumpkin pie spice. Sprinkle mixture evenly over pumpkin mixture in cooker. Drizzle butter over mixture in cooker.
Cover and cook on high-heat setting for 2-1/2 hours. Turn off cooker. If possible, remove crockery liner from cooker and place on a wire rack. Cool, uncovered, for 30 minutes.
To serve, spoon warm pudding into dessert dishes. If desired, top each serving with dessert topping.