Prepare the brownie batter as directed on the box.
Pour into crock pot
In a small bowl whisk together sugar, cocoa powder, and boiling water. Pour gently over brownie batter, not mixing or stirring at all.
Cover and cook on high for 2-3 hours, or until done.
Best served warm.
Notes
To help keep the moisture off the brownies and make sure they aren't soggy, you can place plastic wrap or paper towel over the crockpot, under the lid to catch the water droplets.