1. Beat cake mix, next 5 ingredients, and 1 1/4 cups milk at medium speed with an electric mixer 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. 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. Spoon into dessert dishes, and serve with ice cream, if desired.