01 - Line a 9x13-inch baking pan with parchment paper or lightly grease it.
02 - In a large saucepan over low heat, melt the unsalted butter.
03 - Add mini marshmallows to the melted butter and stir continuously until smooth and fully melted.
04 - Remove the saucepan from heat, then stir in vanilla extract and salt if using.
05 - Add the crispy rice cereal and fold gently to coat all pieces evenly.
06 - Quickly transfer the mixture to the prepared pan and press evenly using a buttered spatula or wax paper.
07 - If desired, sprinkle mini chocolate chips on top and press gently to adhere.
08 - Allow the mixture to cool completely at room temperature, approximately 30 minutes.
09 - Cut into 16 bars and serve.