Butter square 9" x 9" x 2" pan. Heat chips, corn syrup, butter and vanilla in a 3 quart saucepan over low heat, stirring constantly until chips are melted and mixture is smooth. Remove from heat.
Stir in Cheerios and marshmallows until all is well coated.
Press evenly in pan, using the back of a buttered spoon. Let cool.
Heat shortening and chocolate chips in a 1 quart saucepan over low heat, stirring constantly until mixture is smooth.
Spread over cereal. Refrigerate until firm- 1 hour. Remove from refrigerator and let set 10 minutes before cutting into 2" x 1 1/4" squares. Makes 24 bars. Enjoy!