Spray a 9x13-inch pan with cooking spray. Measure 6 cups of rice krispies and set aside. In a large microwave bowl, melt butter, marshmallows, and lime zest together in the microwave. Microwave for 90 seconds, stir, and repeat until mixture is smooth. Add krispies to marshmallow mixture and gently stir until evenly coated. Press mixture into prepared pan. Place white chocolate chips in a ziplock baggie. Place in microwave for 20 seconds. Take out and knead to mix. Repeat until chocolate is melted. Cut off a small piece of the corner of the bag and drizzle over bars with. Sprinkle with crushed graham crackers. I usually place them in the fridge to hurry the process (but if you leave them too long, you will have to let them sit out to soften before cutting). Once chocolate has set, cut into bars and enjoy.