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 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 gluten-free crispy rice cereal.

When all 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 to 20 squares.

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