Step 1: Grease a 9×13 inch baking pan with butter or non-stick cooking spray, or line with parchment paper for easy removal.
Step 2: In a large non-stick pot or Dutch oven, melt the butter over low to medium-low heat. For an enhanced flavor, consider browning the butter slightly until golden and fragrant, taking care not to burn it.
Step 3: Add the mini marshmallows to the melted butter. Stir continuously with a heat-proof spatula until the marshmallows are completely melted and smooth. Ensure the heat remains low to prevent the marshmallows from becoming tough.
Step 4: Once the marshmallows are fully melted, remove the pot from the heat. Stir in the vanilla extract, if using, until well combined.
Step 5: Quickly add the Rice Krispies cereal to the marshmallow mixture. Gently fold and stir the cereal until it is evenly coated with the melted marshmallow. Avoid over-mixing or crushing the cereal to maintain its crispiness.
Step 6: Transfer the mixture to the prepared baking pan. Using a buttered spatula or a piece of wax paper, gently and evenly press the mixture into the pan. Do not press too firmly, as this can make the treats hard and dense. A light touch will result in a chewier texture.
Step 7: Allow the treats to cool completely at room temperature for at least 30-60 minutes, or until firm. Once cooled, use a sharp, buttered knife to cut them into squares. For best results, store in an airtight container at room temperature.