On a parchment-lined baking sheet, place marshmallows. With the rack on the second highest setting, broil for 5-7 minutes until toasted to your liking (note: rotate for even toasting). If you want to garnish your milkshake with toasted marshmallows, you can add them to this step, and set them aside.
Crumble/smash graham crackers finely in a bag. Add to a plate.
In a small bowl, melt the chocolate for 30 seconds, then in 15 second increments until fully melted.
Dip the rim of your milkshake glasses in the chocolate and then into the crushed graham cracker. If desired, drizzle any additional chocolate with a spoon on the inside of the glass and set aside.
Add ice cream, milk, and marshmallows into the blender. Blend until smooth.
Pour into prepared glasses, and garnish with additional toasted marshmallows, if desired.