Step 1: In a large saucepan, melt the butter over low heat. Ensure the heat is kept low so the butter does not brown too quickly.

Step 2: Add the marshmallows to the melted butter and stir continuously over low heat until they are completely melted and smooth. Remove the saucepan from the heat, and stir in the vanilla extract if desired.

Step 3: Add the crispy rice cereal into the marshmallow mixture. Stir gently using a spatula until the cereal is evenly coated and thoroughly combined.

Step 4: Transfer the warm mixture into a greased 9x13 inch baking pan. Using a piece of wax paper or a greased spatula, gently press the mixture evenly and firmly into the pan (do not pack it down too tightly, or the treats will become hard).

Step 5: Allow the treats to cool completely at room temperature (about 30 minutes). Cut them into squares and serve!