In a large bowl, mix the milk and cereal together. Let the mixture sit for 1-2 hours and then strain through a fine mesh strainer gently pressing on the cereal to squeeze out the milk.

Transfer the milk to a blender and add the ice cream. Blend until smooth. Serve in a milkshake glass and top with whipped topping, if desired. Sprinkle with crushed cereal and some of the marshmallows. Serve immediately.