Spray inside of 6-quart slow cooker insert generously with non-stick cooking spray, coating bottom and sides completely.
Add 1 box (about 15.25 ounces) chocolate fudge cake mix, 1 cup milk, ½ cup vegetable oil, and 3 large eggs to large mixing bowl.
Whisk or beat with electric mixer until completely smooth with no lumps, about 1 to 2 minutes.
Pour cake batter into prepared slow cooker insert and smooth top into even layer with spatula.
In separate medium bowl, whisk together 1 box (4-serving size, about 4 ounces) instant chocolate pudding mix with 2 cups cold milk for 1 to 2 minutes until mixture thickens.
Pour thickened pudding mixture evenly over cake batter in slow cooker. Do not stir or mix layers together.
Sprinkle 11 to 12 ounces (about 2 cups) milk chocolate chips evenly over pudding layer. Do not stir.
Fold clean dish towel in half and place directly on top of slow cooker insert, then place lid on top of towel.
Cook on Low for 3 hours to 3 hours 30 minutes, until edges are set and firm but center still has slight jiggle.
Turn off slow cooker and carefully remove ceramic insert from heating base. Set on wire cooling rack or heat-safe surface.
Remove lid and towel. Let cake stand uncovered for 30 minutes.
Scoop portions into bowls, making sure to get both cake and molten sauce from underneath. Top with vanilla ice cream or ice cream of your choice and serve immediately.