To a large bowl, add the cake mix, melted butter, water and eggs. Mix until combined, but don't overmix (some lumps are fine.)
Slice the Butterfinger bars into 1/4 inch pieces, try not to pulverize them, you'll want chunks of it in the batter. Add to the batter and gently stir.
Spray the slow cooker with non-stick spray. Pour in the batter.
Place the lid on the slow cooker and cook on HIGH for 1 hour 45 minutes to 2 hours or until the center is set.
When the cooking time is up. Place the hot fudge jar in the microwave (lid off) for 30 seconds or until melted.
Pour over the cake and spread with a spoon.
Chop up more Butterfinger bars for garnish on top and serve with ice cream.
Video
Notes
Allow the entire cake to cool completely and add the leftovers to an airtight container and store in the refrigerator for up to one week.
Leftover Butterfinger cake is great cold, just as any other cake!