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 the mini marshmallows to the melted butter and stir continuously until completely melted and smooth.
04 - Remove from heat and stir in the vanilla extract and salt if using.
05 - Fold in the crispy rice cereal gently until all pieces are evenly coated with the marshmallow mixture.
06 - Quickly transfer the mixture to the prepared pan and press evenly using a buttered spatula or wax paper.
07 - If using, sprinkle mini chocolate chips on top and press gently into the surface.
08 - Allow the bars to cool completely at room temperature for about 30 minutes to set.
09 - Cut into 16 bars and serve.