While oven is heating, place 10 marshmallows on a baking sheet covered with aluminum foil. Then add 3 graham crackers, chocolate from half a chocolate bar, and 3 marshmallows on top (you should have 3 s'mores). Broil on high for 30-seconds to 1 minute until marshmallow is toasty. Remove from oven and let cool.
In a blender, add vanilla ice cream, whole milk, and 1 chocolate bar. Blend for 10 seconds. Then add in 6 toasted marshmallows, and one s'more. Pulse 5 times until just incorporated.
Drizzle chocolate syrup in glasses, then divide the s'mores milkshake batter between both glasses. Top with 2 toasted marshmallows and one full s'more. Enjoy immediately.