In a microwave-safe medium bowl, add in the water and butter. Microwave for 2 and half minutes.
Once the butter is completely melted, add in the vegetable oil and sour cream. Whisk until sour cream is incorporated.
Add in the cocoa powder and continue to whisk until the mixture is smooth.
Add in the melted baking chocolate and whisk until it's thoroughly incorporated. (To melt the baking chocolate, break the chocolate up into pieces. Place in a microwave-safe bowl, and microwave for 30 seconds. Stir. Repeat microwaving for 30 seconds and stirring afterwards until it's completely melted.)
Add in the vanilla extract and both eggs. Whisk until the eggs are incorporated. Make sure there are no streaks of the egg whites in there. If there is, keep whisking! Set aside.
In a large mixing bowl, add in the sugar, flour, baking soda, and baking powder. Whisk to combine the dry ingredients.
Pour the chocolate mixture into the flour mixture. Stir to combine the chocolate and flour. It will be quite lumpy.
After the flour is stirred in, grab your whisk and whisk until the mixture is smooth. It may take a minute to get it creamy rather than lumpy.
Spray your slow cooker with non-stick cooking spray. I used my 4 QT slow cooker, but a larger one will work. The cooking times will likely be different with a bigger crock pot. It should cook faster.
Pour the batter into the prepared slow cooker.
Add spoonfuls of the hot fudge mixture into the center of the batter. I added all of the fudge sauce.
Top the batter with half of the bag of milk chocolate chips.
Cover the slow cooker with a paper towel and place the lid on top.
Set the crock pot to high and cook for 4 hours or until the center of the cake is set up. You can use a toothpick and make sure the top is set up by inserting it. Moist crumbs are good, cake batter means it's still raw.
After the cake is done, add the remaining milk chocolate chips. Scoop and serve with ice cream.
Pro Tip: I removed the paper towel after 2 hours. Totally not necessary though.