Spray 5-quart oval slow cooker with cooking spray. In large bowl, beat cake mix, chocolate pudding mix, sour cream, butter, vanilla, 1 1/4 cups of the milk and the eggs with electric mixer on medium speed 2 minutes, scraping sides of bowl as needed. Stir in 1 cup of the toffee bits. Pour batter into slow cooker.
In 2-quart saucepan, 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.
Cover; cook on Low heat setting 4 hours 30 minutes 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. Garnish with whipped topping and remaining toffee bits.