Line a 9x9 inch pan with aluminum foil and spray with non-stick spray.

In a medium-large pot, melt the butter over medium-low heat. When the butter is melted, reduce the heat to low and add 5 cups of the mini marshmallows. Stir frequently until the marshmallow are melted.

Remove from the heat and stir in the vanilla extract and the remaining 1 cup of marshmallows. Stir to melt the marshmallows a bit (chunks of marshmallow are good) then stir in the chocolate crispy rice cereal.

When all of the cereal is coated with the marshmallow, transfer to the prepared pan. Let it sit in the pan to cool for a minute or two then use a spatula or your hands to press into the pan. (I find that spraying my clean hands with non-stick spray or wetting them with a small amount of water works best.)

Cool completely then cut into 16-20 squares.

Store in an air-tight container at room temperature for up to 4 days.