Coat the inside of a 6-7 quart slow cooker with nonstick cooking spray.

Following the box instructions, combine brownie mix with eggs, oil, and water (or as directed on the brownie mix package). Pour the prepared batter evenly into the greased slow cooker.

In a separate medium bowl, whisk together the pudding mix and milk until smooth, then carefully pour this mixture over the brownie batter in the slow cooker.

Lay a paper towel across the top of the slow cooker before covering with the lid, ensuring it will catch condensation and prevent it from dripping into the pudding.

Cook on high for 2 to 3 hours, beginning to check around the 2-hour mark. The edges should appear slightly dry and firm, while the center may remain jiggly and moist. Avoid relying on the toothpick test, as some pudding will stay moist on top. Most slow cookers will finish the dessert around the 3-hour mark; avoid overcooking to keep the brownie base moist.

Serve warm, garnished with ice cream or whipped cream if desired. Store any leftovers in an airtight container in the refrigerator for up to 3 days.