1. Beat cake mix, next 5 ingredients, and 1 1/4 cups milk at medium speed with an electric mixer (used my Kitchen Aid) 2 minutes, stopping to scrape down sides as needed. Pour batter into a lightly greased 4-qt. slow cooker.
2. Cook remaining 2 cups milk in a heavy nonaluminum saucepan over medium heat, stirring often, 3 to 5 minutes or just until bubbles appear (do not boil); remove from heat.
3. *Sprinkle cook-and-serve pudding mix over batter. Slowly pour hot milk over pudding. Cover and cook on LOW 3 1/2 hours.
4. (optional if using pecans) Meanwhile, heat pecans in a small nonstick skillet over medium-low heat, stirring often, 3 to 5 minutes or until lightly toasted and fragrant.
5. Turn off slow cooker. Sprinkle cake with pecans, marshmallows, and chocolate morsels. Let stand 15 minutes or until marshmallows are slightly melted.