Line a 9-inch (23 cm) square pan with parchment leaving overhang for easy removal. Tear a small second sheet of parchment and set aside.

In a large saucepan or Dutch oven, melt the butter over low heat. Add marshmallows and stir until melted and smooth. Remove pot from heat. (Alternatively, in a large microwave-safe bowl, microwave the butter and marshmallows on High in 30-second intervals, stirring in between, until melted and smooth.)

Add the orange juice concentrate and zest, stirring until well incorporated. Stir in the rice cereal, mixing to coat well.

Working quickly, transfer the mixture into the prepared pan, place the second parchment sheet on top and press the mixture flat with the palm of your hand. Be sure to press the mixture into the edges of the pan as well.

Let cool to room temperature, about 30 minutes. Remove from pan using the parchment overhang, and cut into squares.

Store in an airtight container at room temperature.