Grease a 9 x 9 inch (23 x 23 cm) pan with butter.

Melt the 4 tbsp of butter in a large pot on medium low heat. Add the marshmallows and stir until they are all melted. Now add the vanilla extract and stir until it is well incorporated. Finally toss in the cereal. At this point you can add any other ingredient that you want (M&M's, peanut butter, chocolate chips, sprinkles). Stir until the cereal is well coated.

Pour mixture into the prepared pan and press lightly with clean hands to fill in the pan. You can also grease your hands with a little bit of butter to make it easier. Let it cool completely at room temperature and cut into 16 small squares.