Line a 9x13-inch baking pan with parchment paper and lightly grease it. Set aside.
In a large saucepan over medium-low heat, melt the butter completely. Don't let it brown or bubble aggressively — you just want it melted.
Add the marshmallows and stir constantly until they're mostly melted and the mixture looks smooth and glossy, about 3-4 minutes. If some clumps remain at this stage, that's fine — they'll incorporate when you add the cereal.
Remove from heat and stir in the lemon zest and lemon juice until combined. The mixture will look a little thin but don't worry; it thickens as it cools slightly.
Add 4 cups of Rice Krispies and fold gently with a spatula until coated. Then add the remaining cereal gradually, folding just enough to incorporate — you want some clusters for texture, not mushy uniformity.
Press the mixture into your prepared pan using lightly greased hands or a greased spatula. Don't press too hard; compacting it makes dense treats rather than light ones.
Let cool at room temperature for 20-30 minutes before cutting. For cleaner cuts, use a sharp knife and wipe between slices. Cut into squares about 1.5 inches wide.
Store in an airtight container at room temperature for up to 4 days, or refrigerate for up to 2 weeks. They'll firm up more in the fridge but lose some chewiness.
