Grease a 9x9-inch baking pan with cooking spray or use a little butter to prevent sticking.
In a large pot, melt the unsalted butter (or vegan butter) over low heat.
Add the mini marshmallows to the melted butter and stir constantly until the marshmallows are completely melted and smooth.
Remove the pot from the heat and stir in the vanilla extract and a pinch of salt, if using.
Quickly add the crispy rice cereal to the melted marshmallow mixture and gently fold it in until the cereal is fully coated.
Transfer the mixture to the prepared baking pan.
Use a greased spatula or your hands (lightly greased) to press the mixture evenly into the pan.
Allow the treats to cool and set at room temperature for about 30 minutes.
Once the treats are completely cooled and set, use a sharp knife to cut them into squares or rectangles.
Enjoy your tasty vegetarian rice crispy treats with melted chocolate for an extra yummy treat!