"Growing up for some reason we referred to these as 'Oh Henry Squares'. I've no idea why, but I have not served them without people requesting the recipe and rave reviews. I don't know the origin. It's just been passed on through my family."
Cook and stir peanut butter, brown sugar, and corn syrup together in a large pot over medium heat just until warm and well blended, about 3 minutes; remove from heat. Stir cornflakes cereal and crispy rice cereal into peanut butter mixture; press cereal mixture evenly into the prepared pan.
Whisk confectioners' sugar and custard powder together in a bowl. Whisk 1/2 cup melted margarine gradually into custard mixture. Stir warm water into custard mixture to moisten to a spreadable consistency; spread evenly over cereal mixture.
Melt chocolate with remaining 2 tablespoons margarine in a microwave-safe bowl, about 1 minute. Pour chocolate mixture over custard layer. Refrigerate cookies until chocolate is hardened, about 30 minutes; cut into 1-inch squares.