Line a 9x13 baking pan with aluminum foil, leaving some overhang for easy removal. Spray the foil with nonstick cooking spray or grease with butter and set aside.
Place a large saucepan over medium heat, add the sliced butter to the saucepan and allow to melt completely. While the butter is melting, remove two cups (110 grams) of mini marshmallows from the package and set aside for later.
Once the butter is melted, add the mini marshmallows (minus the two cups you set aside). Continue stirring the mixture until the marshmallows are completely melted and smooth. Remove from the heat and stir in the vanilla extract.
Add half of the cereal and mix until well combined, then stir in the remaining cereal and continue mixing until fully combined. Stir in the reserved two cups of marshmallows until just combined.
Scoop the mixture into the prepared baking pan and press it down firmly into one even layer.
Allow to cool for at least 45 minutes, then remove from the pan, slice into bars, and enjoy!