Gather all your ingredients and tools.
In a large pot, melt the butter over low heat.
Add the mini marshmallows to the melted butter.
Stir continuously until the marshmallows are completely melted.
Remove from heat and stir in the vanilla extract (if using).
Gradually add the Rice Krispies cereal to the melted marshmallow mixture.
Mix until all the cereal is evenly coated.
Grease a 9x13-inch baking pan for easy removal.
Pour the mixture into the prepared pan and press down firmly.
Allow the treats to cool completely before cutting into squares.