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