Add butter to a pot and melt over medium heat.
Once the butter is melted, add mini marshmallows. Continue to stir and cook over medium heat.
When the marshmallows are fully melted and incorporated into the butter, remove the pot from the heat. Stir in the vanilla extract.
Stir in the rice krispies cereal.
Stir until all of the cereal is well coated in the melted marshmallow mixture.
Line an 8×8 dish with waxed paper or foil coated lightly with oil. Press the rice krispie mixture into the dish using a lightly oiled spatula. Avoid packing it down too firmly.
Allow to cool at room temperature or in the refrigerator until firm, then slice into nine pieces.