PREPARE the casserole dish by lining with aluminum foil. This will make it easier to lift out the Rice Krispie Treats for cutting.
PLACE 2/3 of a package of Oreo cookies into a large ziplock bag; seal and crush into large chunks. Measure out 6 cups of Rice Krispies cereal into a large bowl and mix in Oreo chunks.
MELT butter, mini marshmallows, and vanilla extract in a large pot over medium-low heat, stirring constantly. Remove from heat when marshmallows are almost completely melted; continue stirring until smooth.
POUR Oreo/Rice Krispies cereal into pot and stir to combine. Once combined, pour and press into prepared casserole dish.
MELT semi-sweet and white baking chocolates in two small, separate ziplock bags. (To melt in the microwave, cook for 30 seconds, squeeze the bag with your hands to mix, and repeat in 20 second intervals until melted. You can also place the bags into hot water to melt the chocolate.) Once melted, use your fingers to push all the chocolate to one corner of the bag and snip a small part of the corner off with scissors. Use this to drizzle the chocolates (one at a time) onto the Rice Krispie Treats in a zip-zag pattern.