Step 1: Grease a 9x13 inch baking dish with butter or non-stick cooking spray, or line with parchment paper for easier removal.
Step 2: In a large, heavy-bottomed pot or Dutch oven, melt the butter over low heat. Keep the heat low to prevent burning.
Step 3: Add the marshmallows to the melted butter. Stir constantly until the marshmallows are completely melted and smooth. This takes about 5-7 minutes. Remove the pot from the heat as soon as they are fully melted.
Step 4: Quickly stir in the vanilla extract, if using.
Step 5: Add the Rice Krispies cereal to the marshmallow mixture. Using a buttered spatula or spoon, gently fold the cereal into the marshmallow mixture until it is evenly coated. Do not press too hard or overmix.
Step 6: Transfer the mixture to the prepared baking dish. Using a buttered spatula, a piece of parchment paper, or damp hands, gently press the mixture evenly into the pan. Do not compact it too tightly.
Step 7: Allow the treats to cool completely at room temperature for at least 30-60 minutes before cutting into squares. This can be sped up by refrigerating for 15-20 minutes.
Step 8: Once cooled, cut into squares and serve. Store any leftovers in an airtight container at room temperature for up to 3 days.