Fill a large bowl with 6 cups of Rice Krispies cereal. 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!
Add the 10 ounce bag of jumbo 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 Rice Krispies cereal. Use a large wooden spoon or rubber spatula to fold the ingredients together until evenly distributed.
While the mixture is still warm, add the mini marshmallows and fold the two together again (this part can be omitted if you don’t prefer full marshmallows in your Rice Krispies).
Transfer half of the mixture to the prepared baking dish and press into the pan evenly. Then pour the warm dulce de leche on top of the Rice Krispies mixture and spread evenly.
Pour the second half of the rice krispy mixture on top and press into an even layer.
Let the treats sit at room temperature for about an hour before eating.
Cut and enjoy or store in a ziploc bag or a tightly sealed container for later. Enjoy!