Begin by lining a 9x9-inch pan with parchment paper. Allow the paper to hang over two of the edges for easy removal. Spray with baking spray.
In a large microwave-safe bowl, add the butter. Microwave for about 45 seconds to start melting it.
Set about 2 cups of the marshmallow to the side, then add the rest to the butter.
Return the bowl of butter and marshmallows to the microwave. Continue to microwave at 30-second intervals, stirring between, until the marshmallows are completely melted and the mixture is smooth.
Add in the salt and vanilla extract and stir to combine.
Stir in the cereals and mix gently to start combining. Then, add in the reserved 2 cups of marshmallows. Continue to gently mix until combined.
Transfer this mixture to your prepared pan. Spray your fingertips with a bit of baking spray and then press the bars evenly, but gently into the pan.
Allow the treat mixture to cool and set. Remove from the pan and cut into bars.
Notes
I think these are best enjoyed the same day, but you can pack and store them in an airtight container for up to two days.