Combine brown sugar, sugar and corn syrup and mix well. Microwave on high for 2 minutes.
Remove bowl from microwave and stir mixture. Return to microwave and microwave on high for another 2 minutes, or until mixture begins to bubble up.
Remove from microwave and add 1-1/2 cups peanut butter. Stir until mixture is smooth and combined. Add cereal and stir to coat. Pour and press into greased 13×9 pan, let cool for 20 minutes before adding frosting.
Combine chocolate chips and 1/3 cup peanut butter in small microwave safe bowl. Heat on high at 30 second intervals stirring each time (about 1 minute 15 seconds). Stir until smooth, then spread over cereal mixture. (Do Not Refrigerate) let harden at room temp for 1 hour before serving.