Step 1: Prepare a 9×13-inch baking pan by lightly greasing it with butter or non-stick cooking spray, or by lining it with parchment paper, leaving an overhang on the sides for easy lifting.
Step 2: In a large, heavy-bottomed pot or Dutch oven, melt the unsalted butter over low heat. Once melted, add the mini marshmallows. Stir constantly until the marshmallows are completely melted and smooth. Immediately remove the pot from the heat.
Step 3: Stir in the pure vanilla extract and the optional salt into the melted marshmallow mixture until well combined.
Step 4: Gently pour the crispy rice cereal into the marshmallow mixture. Using a buttered spatula or spoon, carefully fold the cereal into the marshmallow until it is evenly coated. Work quickly but gently to avoid crushing the cereal.
Step 5: Transfer the mixture to your prepared 9×13-inch pan. Lightly butter your hands or a piece of parchment paper, or spray your spatula with non-stick spray, and gently press the mixture evenly into the pan. Do not pack it down too firmly.
Step 6: Allow the rice krispie treats to cool completely at room temperature for about 1-2 hours. Once cooled, use the parchment paper overhang to lift the entire slab out of the pan, then cut into 16 squares. Store in an airtight container at room temperature.