Line a 9x13-inch baking pan with parchment paper and lightly grease with non-stick spray.

In a very large bowl, toss together the crisped rice cereal and graham cracker cereal.

In a large pot over medium-low heat, melt the butter completely.

Reduce heat to low. Add vanilla extract and 12 ounces of the mini marshmallows. Stir constantly until completely melted and smooth.

Remove from heat immediately and pour the marshmallow mixture over the cereals. Gently fold until coated.

Let cool for 60 seconds, then gently fold in the remaining 4 ounces of mini marshmallows and the chopped milk chocolate.

Transfer the mixture into the prepared pan. Using greased hands or a spatula, very lightly press the mixture into an even layer. Do not pack firmly.

Allow to set at room temperature for at least 1 hour before slicing into squares and serving.