Spray a 8x8 or (9x13 for thinner bars) baking dish with cooking spray. Measure out your 5 cups of cereal so it's ready.
Melt the butter in a large pot over medium-low heat. Add the marshmallows. When almost melted stir in 1½ cup white chocolate chips. When they are melted, quickly add the 5 cups rice crispy treats. Stir till combined.
Press mixture into prepared baking dish (I always spray my hands with cooking spray so it doesn't stick.) Melt remaining ½ cup chocolate chips in microwave. Put in a zip top bag, cut the corner and drizzle over the top of the crispy treats. When chocolate is set on top, slice and enjoy!
Notes: I used a 9x13 dish. But an 8x8 would make nice thick bars. Either size works.
Recipe by Baked in AZ at http://bakedinaz.com/white-chocolate-rice-crispy-treats/