Place puffed wheat in a large bowl; set aside. Grease a 9-inch square pan.
Grease the rim of a medium saucepan to prevent boil-over. Place cocoa powder, corn syrup, brown sugar, and butter or margarine in the saucepan. Cook over medium heat, stirring often until the mixture comes to a full boil. Allow to boil for 1 minute, and then remove from heat.
Pour chocolate mixture over puffed wheat and stir until puffed wheat is evenly coated. Using a buttered spatula, press the mixture into the prepared pan. Allow to cool, then cut as desired. Wrap squares individually, or store in an airtight container.