My old standby, because I've found no better: One bowl brownie recipe from the back of the Baker's Unsweetended Chocolate box. (Actual use of Baker's unsweetended chocolate is optional.)
The trick is always to throw them in the freezer or refridgerator before you cut them or their inner gumminess will drag along with the knife and mess up your neat edges. I didn't have time to fully cool them, thus, crumbly they are. I might even like them better, all rustica.