Spray the inside of a slow cooker bowl with nonstick cooking spray and set aside.
In a medium bowl, add flour, pudding mix, sugar, baking powder and salt. Whisk together until well combined. Make a well in the center.
To the center of the well in the dry ingredients, add milk, vegetable oil and vanilla. Stir until combined and batter is smooth.
Stir in butterscotch chips and then pour batter into slow cooker.
In a small bowl, combine butterscotch sauce, sugar and boiling water. Stir until smooth and then slowly and evenly pour over cake batter.
Cover and cook on high for 2.5 hours. When cooking time is done, leave lid on and let sit for 30 minutes.
To serve, remove a piece of the cake and then add vanilla ice cream or whipped cream if desired. Spoon the sauce from the bottom of the slow cooker to drizzle over top.