Grease a 9x13 inch baking dish with non-stick cooking spray. This prevents the treats from sticking and makes it easier to remove them later.

In a large saucepan over low heat, melt the unsalted butter. Stir gently to ensure it doesn't burn.

Once the butter is melted, add the marshmallows to the saucepan. Continue stirring until the marshmallows are completely melted and the mixture is smooth.

If using, stir in the vanilla extract for added flavor.

Remove the saucepan from heat. Quickly add the Rice Krispies cereal to the melted marshmallow mixture. Stir gently but thoroughly until all the cereal is evenly coated with the marshmallow mixture. This step is crucial to ensure every piece is covered and the treats hold together.

Using a buttered spatula or your hands (lightly greased), press the mixture evenly into the prepared baking dish. Make sure to pack it down firmly to avoid crumbling later.

Allow the treats to cool at room temperature for about 30 minutes. Once set, cut into squares or rectangles using a sharp knife. Wipe the knife with a damp cloth between cuts to prevent sticking.