On a foil-lined baking sheet, toast marshmallows under a broiler, turning as necessary, until all sides are golden brown. Keep a close eye on them as they can burn rather quickly.
Just as the marshmallows are finishing up, put ice cream and milk in the blender. Combine briefly, then add toasted marshmallows and blend until incorporated. Don’t overblend or it will be too melty. If this happens, adding an extra scoop of ice cream will thicken it up a bit.
Pour shake into a glass and top with some whipped cream and some crushed graham crackers. If you have some leftover marshmallows, you can toast those up and use that as a garnish as well.
This is a pretty rich shake, so a small glass for each person will be plenty. Double recipe and repeat as necessary (you should get about 12-16 ounces per batch).