Liberally grease large slow cooker with cooking spray.
In large bowl, combine brownie mix, oil, 1/4 cup water, and eggs until well mixed.
Pour brownie batter into prepared slow cooker.
In same bowl (no need to wash), whisk together hot fudge sauce and hot water until smooth.
Carefully pour fudge mixture over brownie batter.
Cover and cook on HIGH for 2 to 2½ hours.
Brownies are done when edges are set but center is still gooey.
Let rest 5 minutes before serving.
Scoop into bowls and serve warm with desired toppings.