Line a 9x13 inch baking pan with parchment paper, leaving an overhang on two sides. Lightly grease the parchment.

Optional: In a large pot, melt 1 tbsp butter over low heat. Add lemon zest and toast for 60-90 seconds until fragrant. Remove zest to a bowl.

In the same pot, melt remaining 5 tbsp butter over low heat. Add mini marshmallows and salt. Stir constantly until completely melted and smooth.

Remove pot from heat. Immediately stir in the toasted lemon zest and 1 tbsp lemon juice.

Quickly add the crispy rice cereal. Fold gently until all cereal is evenly coated in the marshmallow mixture.

Press the mixture firmly and evenly into the prepared pan using buttered hands or parchment paper. Let cool at room temperature for 1-2 hours until completely set.

Make the glaze: Whisk sifted powdered sugar, 2 tbsp lemon juice, corn syrup (if using), and a pinch of salt until smooth. Adjust consistency with more juice or sugar as needed.

Lift the set slab from the pan using the parchment. Pour glaze over the center and spread to the edges. Garnish with extra zest if desired. Let glaze set for 15-20 minutes.

Using a sharp knife, cut into 16 squares. Clean the knife between cuts for neat edges.