Spray a 6-7 quart slow cooker with nonstick cooking spray.
Prepare brownie mix as directed on the back of the box. Pour into prepared slow cooker.
Whisk pudding mix and milk in a medium sized bowl until smooth. Pour carefully over the brownie mix in the crockpot.
Cover the crock pot with a paper towel and place the lid on top (to catch the condensation). Cook on high power 2-3 hours. It will probably take at least 2 1/2 hours, but start checking it after 2. Some of the pudding stays on the top and stays wet looking, so you cannot trust the toothpick test and it will stay somewhat jiggly until it’s done. Watch the edges – when they look somewhat dry and done, then the pudding is ready. Baking time will be different for every slow cooker, but after 3 hours on high it should be done. You don’t want to overcook it or the brownie (which is on the bottom) will be dry.
Serve warm with ice cream or whipped cream. Store in an airtight container in the regfrigerator for up to 3 days.