Fill a large bowl with the Rice Krispies cereal and the Lucky Charms Cereal (but not the lucky charms marshmallows). Set aside.
Spray a 9x9 baking dish with nonstick spray. Set aside.
In a large pot add the unsalted butter. Stir the butter continuously over low heat for about 10 minutes until the foam subsides, the milk solids gain a brown color, and the butter begins to smell nutty. You have now browned your butter! If you want a faster recipe, you can skip the browning step and simply melt the butter and move on to step 4 right away.
Add the 10 ounce bag of marshmallows to the pot and continuously stir until they completely melt.
Add in the vanilla extract and sweetened condensed milk, stirring everything once again until combined.
Remove the marshmallow mixture from the heat and pour over the bowl filled with the cereals. Use a large wooden spoon or rubber spatula to fold the ingredients together until evenly distributed.
While the mixture is still warm, add the Lucky Charms marshmallows and fold the two together again.
Transfer the mixture to the prepared baking dish and press into the pan evenly. Let the treats sit at room temperature for at least an hour before eating.
Cut and enjoy or store in a ziploc bag or a tightly sealed container for later. Enjoy!