Grease the inside of a 6-quart slow cooker with butter.
In a large bowl, whisk together the oil, ¼ cup of water and two eggs. Add the brownie mix and stir until combined. Add the chopped Reese's cups and stir until combined.
Spread the brownie batter evenly in the bottom of the greased slow cooker.
In a separate bowl (or glass measuring cup), combine the hot fudge, peanut butter and remaining ½ cup of hot water. Whisk until smooth. Pour the sauce over the brownie mix in the slow cooker.
Place the lid on the slow cooker and cook on HIGH power for 2 to 3 hours (or until center of brownie is just set). Serve warm topped with vanilla ice cream.
The calories shown are based on the brownies being divided 10 ways, with 1 serving being 1/10 of the recipe. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate.