I found two slightly different versions for you. The first uses only the ingredients you listed; the second includes butter and more white syrup than the first, and uses plain Rice Krispies with additional cocoa powder. Hope one of them is the one you need!

COCOA PEANUT SQUARES

1/3 cup light Karo corn syrup
1/2 cup brown sugar, firmly packed
1/2 cup peanut butter
3 cups Kellogg's Cocoa Krispies cereal

Butter a 13x9-inch dish.

Combine Karo and sugar in 3 quart pan and cook over medium heat, stirring frequently until mixture bubbles around edge of pan, remove from heat.

Stir in peanut butter until completely mixed.

Add Cocoa Krispies, stirring until well coated. Press into prepared dish.

Cool and cut into squares.

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CANDY COCOA KRISPIES

1 cup brown sugar
1/2 cup light corn syrup
1/2 cup butter
2 tbsp. cocoa
1 tbsp. vanilla
5 cups Rice Krispies cereal

Butter a 13x9-inch dish.

Combine all ingredients except cereal in a saucepan and boil for 3 minutes.

Add cereal and pat into prepared dish.

Cool and cut into 30 squares.