Butter a 12- by 18-inch sheet pan and set aside.
In a large nonstick skillet, combine the butter, sugar, water and corn syrup. Cook until the mixture turns a dark caramel color. Add the vanilla and cashews.
Pour onto the buttered sheet pan and spread evenly. Immediately top with the broken chocolate bars. When the chocolate melts, spread the mixture evenly with a knife or spatula.
Cool for several hours or overnight, then break into small pieces. (Other types of nuts and fancy chocolate may be substituted, but Lainey has found that cashews and “regular old Hershey’s bars” are everybody’s favorite.)