Line a 9x13" baking dish with parchment paper or spray with nonstick cooking spray.
Place 16 ounces of marshmallows, butter and lemon extract in a large microwaveable bowl.Heat in the microwave for 60 seconds. Stir. Heat in additional 30 second increments, stirring in between, until melted and smooth.
Stir in cereal, chopped cookies and remaining 4 ounces of mini marshmallows.
Press mixture into prepared baking dish. Spray your hands with non stick cooking spray or grease with a small amount of butter to prevent the mixture from sticking to your hands.Allow lemon Rice Krispies treats to cool for 30 minutes then cut into squares.
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Notes
Stove top directions: heat butter, marshmallows, and lemon extract in a large pot on medium low heat and stir until melted and smooth. Stir in cereal, cookies, and remaining mini marshmallows using a wooden spoon. Prepare the rest the same as above.