Combine brown sugar, butter, corn syrup, and cocoa in a saucepan. Bring to a boil; cook, stirring constantly, until sugar is dissolved, about 2 minutes. Remove from heat. Stir in vanilla extract.
Pour sugar mixture over puffed wheat cereal in the bowl; stir until cereal is evenly coated. Spread evenly into the lined pan. Cool until set, about 30 minutes. Cut into squares.
