Grease or line a 9x13-inch pan with parchment paper. Parchment makes the bars easy to lift out and cut for serving.

Melt butter in a large saucepan over low heat. Use a 6-quart pot or larger saucepan as this will make stirring much easier when adding the cereal.

Add in marshmallows and stir until fully melted. Once melted, remove the marshmallow mixture from heat and stir in vanilla extract or vanilla bean paste.

Add in cornflake cereal and rice krispies cereal. Stir until thoroughly mixed. Alternately, mix everything together in a large bowl if the saucepan is too small.

Pour mixture into prepared pan. Press down to create a flat, even layer of bars. Pro tip: use a cupcake liner as a barrier between your hand and the bars so they don't stick. Press them down tightly. Let sit to cool for one hour as the marshmallow mixture hardens slightly to set. Cut and serve.