Lightly butter an 8 × 8-inch baking pan or line it with parchment paper, leaving a little overhang on the sides for easy removal. Measure the crispy rice cereal and lemon zest so they are ready to add once the marshmallow mixture is melted.

Place the butter in a large saucepan over low heat. Stir gently until the butter melts completely, then add the marshmallows and lemon extract. Continue stirring until the marshmallows are almost melted and the mixture is glossy. Leaving a few small marshmallow pieces unmelted helps create soft, gooey pockets throughout the treats.

Remove the saucepan from the heat and immediately stir in the crispy rice cereal and lemon zest. Mix gently but thoroughly until the cereal is evenly coated with the lemon marshmallow mixture.

Transfer the mixture to the prepared baking pan. Using a lightly buttered spatula or a piece of parchment paper, gently press it into an even layer. Avoid packing the mixture too firmly, as this can make the finished treats dense and hard.

Allow the mixture to cool at room temperature for about 20–30 minutes, or until firm enough to slice cleanly. Lift it out of the pan using the parchment overhang, then cut into 9 large squares or smaller bars as desired.

Serve the lemon Rice Crispy treats at room temperature while they are soft, chewy, and crunchy.