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 completely melted and smooth.
04 - Remove the mixture from heat and stir in vanilla extract and sea salt, if using.
05 - Quickly add crispy rice cereal and mix gently until fully coated with the marshmallow mixture.
06 - Transfer the coated cereal mixture to the prepared pan and press evenly with a buttered spatula or wax paper, avoiding compacting too firmly for optimal texture.
07 - Allow to cool at room temperature for at least 30 minutes, then cut into 12 squares and serve.