Whisk pudding mix with cold milk for 2 minutes and refrigerate until thickened, about 5 minutes.
Beat heavy cream, powdered sugar, and vanilla to stiff peaks.
Spoon a layer of crushed graham crackers into the bottom of 6 glasses.
Add a layer of chocolate pudding, then a layer of whipped cream, then a few mini marshmallows.
Repeat the layers until glasses are filled, finishing with whipped cream and mini marshmallows on top.
Drizzle with chocolate syrup and serve immediately or refrigerate up to 24 hours.