Melt the butter in a large saucepan over medium heat. Continue cooking, stirring occasionally, until the butter turns golden brown and smells nutty. Be careful not to burn it.
Add the marshmallows to the saucepan with the browned butter. Stir constantly until the marshmallows are completely melted and smooth.
Remove the saucepan from the heat and stir in the vanilla extract.
Add the crispy rice cereal to the marshmallow mixture. Stir gently until the cereal is evenly coated.
Lightly grease a 9x13 inch baking pan with butter or cooking spray. Pour the mixture into the prepared pan and press it down gently with a spatula or buttered hands. Do not press too firmly, or the treats will be hard.
Let the treats cool for at least 15 minutes before cutting into squares. Enjoy!
Notes
For extra flavor, you can add a sprinkle of sea salt on top before the treats set. For softer treats, use slightly fewer cereal. For crisper treats, use slightly more.