In a large saucepan, heat granulated sugar and corn syrup just until mixture boils on edge; cook and stir 1 minute more. Remove from heat. Stir in peanut butter until melted. Stir in rice cereal and cornflakes until coated. Press into bottom of a 15x10x1-inch baking pan; set aside.
In a medium saucepan, melt the 3/4 cup butter. Stir in powdered sugar, vanilla pudding mixes, and enough of the milk to make spreading consistency. Spread over cereal layer; set aside.
In a small saucepan, heat chocolate pieces and the 1/2 cup butter over low heat, stirring constantly until melted. Spread over pudding layer. Cover and chill in the refrigerator for at least 1 hour or until set. To serve, cut into bars. Store leftover bars in the refrigerator for up to 2 days. Makes 64 bars.