Heat syrup and sugar, stirring continuously, until mixture comes to a boil.
Remove from heat; add peanut butter and stir until smooth. Pour mixture over cornflakes in a large bowl. Mix until cereal is thoroughly coated.
Spread in a buttered 9x13-inch pan and press down lightly with buttered hands. Allow to cool.
Place chocolate in the top of a double boiler over simmering water. Stir frequently, scraping down the sides with a rubber spatula to avoid scorching, until chocolate is melted, about 5 minutes. Pour or spread melted chocolate on top of the cornflake mixture. Allow to harden and cut into squares.
