In a large bowl, combine the brownie mix, oil, 1/4 cup water, and eggs until combined. Pour into the prepared slow cooker.
Whisk together the hot fudge sauce and hot water. (Feel free to use the same bowl you mixed the brownie mix in without cleaning it out). Pour over the brownie batter. Top with the lid and cook on HIGH for 2 to 2 1/2 hours.
The brownies will be gooey in the center at the end of cooking (though the edges should be fully set). Scoop out and serve with a spoon and top with ice cream.