1. Beat cake mix, chocolate pudding mix, sour cream, butter, vanilla, 1 1/4 cups milk, and eggs in a large bowl at medium speed of an electric mixer 2 minutes, scraping down sides of bowl as needed. Stir in 1 cup toffee bits. Pour batter into a lightly greased 5-qt. oval-shaped slow cooker.
2. Heat remaining 2 cups milk over medium heat 3 to 5 minutes, stirring frequently, until hot and bubbly. Remove from heat. Sprinkle butterscotch pudding mix over batter in slow cooker. Slowly pour hot milk over pudding.
3. Cover and cook on Low 4 1/2 hours or until edge of cake is set at least 2 inches from edge of slow cooker but center still jiggles slightly when moved. Turn off slow cooker. Let stand 15 minutes. Top with whipped topping and remaining toffee bits.