Line an 8x8 or 9x9-inch baking dish with parchment paper and lightly grease with butter or nonstick spray.
In a large microwave-safe bowl, add butter and marshmallows. Microwave on high for 45 seconds, stir, then heat for another 30 seconds or until melted and smooth.
Stir in vanilla extract if using.
Quickly fold in Rice Krispies cereal and 1 ½ cups of crushed Oreos (make sure to save the other ½ cup for topping).Stir until well combined.
Transfer the mixture to the prepared pan and gently press it down with a greased spatula (avoid pressing too hard to keep them soft and chewy).
Sprinkle the remaining ½ cup of crushed Oreos over the top and press them in slightly.
Let the treats cool at room temperature for about 30 minutes before slicing into squares.
Notes
Use low heat. Melting the marshmallows slowly is the key to soft and gooey crispy treats.
Crush the Oreo cookies to your liking. For a more intense cookies & cream flavor, finely crush them; for more crunch, leave some bigger chunks.
Include mini marshmallows for extra gooeyness. Just stir ½ cup of whole mini marshmallows into the mixture before pressing into the pan.
Use parchment paper when pressing the mixture into the pan. It prevents sticking and helps you to spread the mixture out evenly.
Let them cool completely before cutting. This is the best way create clean, sharp squares.
For an extra rich cookies and cream dessert, drizzle the top of the squares with melted white chocolate!