Spray the inside of your slow cooker with cooking spray.
In a large bowl, combine the chocolate cake mix, 1 ½ cups of milk, vegetable oil, and eggs. Use a handheld or stand mixer to beat the ingredients until smooth and well-mixed, about 2 minutes.
Pour the cake batter into the bottom of the slow cooker.
In a medium bowl, mix the chocolate pudding mix and 3 cups of milk according to the package directions.
Pour the prepared chocolate pudding mixture on top of the cake batter in the slow cooker.
Remove the lid from the can of chocolate frosting and microwave it for 10-15 seconds until it becomes pourable. Evenly pour the frosting over the pudding layer.
Sprinkle the milk chocolate chips on top of the mixture in the slow cooker.
Place the lid on the slow cooker and cook on high for 3 to 3 ½ hours (check at 2 hours for doneness.) Alternatively, you can cook on low for 4 to 5 hours(check at 3 hours for doneness.) The cake is done when a knife inserted into the middle of the cake comes out clean. Make sure not to pierce through to the pudding layer.
Once cooked, remove the slow cooker lid and serve the chocolate lava cake hot. It pairs well with a scoop of vanilla ice cream on top.