Prepare the baking dish by lining it with parchment paper or greasing it with a bit of butter.
In a large microwave-safe bowl, combine the butter and mini marshmallows. Microwave on high for 1 minute, then stir.
Continue microwaving in 30-second intervals, stirring in between, until the mixture is smooth and fully melted. Alternatively, melt on the stovetop over low heat.
Once melted, stir in the vanilla extract and a pinch of salt if desired.
Gradually add the Rice Krispies cereal to the melted marshmallow mixture, gently folding until evenly coated.
Pour the mixture into the prepared baking dish. Press down evenly using a greased spatula or the back of a spoon.
Allow the Rice Krispie treats to cool at room temperature for about 30 minutes, then cut into 12 square pieces.
Serve and enjoy plain or with optional mix-ins like chocolate chips or sprinkles.