01 - Lightly grease a 9x13-inch baking dish with butter or nonstick spray.
02 - In a large saucepan, melt the unsalted butter over low heat.
03 - Add mini marshmallows to the melted butter, stirring constantly until completely melted and smooth. Remove from heat.
04 - Stir in vanilla extract and a pinch of salt if using.
05 - Immediately fold in puffed rice cereal gently until all pieces are evenly coated.
06 - Transfer the mixture to the prepared dish and press evenly with a buttered spatula or wax paper without compacting too firmly.
07 - Allow to cool at room temperature for at least 1 hour before cutting into 16 squares and serving.