Whisk pudding mixes with cold milk for 2 minutes until thickened and set aside.
Beat heavy cream, powdered sugar, and vanilla to stiff peaks, then fold half into the pudding mixture.
Spread a thin layer of pudding mixture on the bottom of a 9x13 inch dish, then layer graham crackers in a single layer on top.
Spread half the remaining pudding mixture over the crackers, then drop spoonfuls of marshmallow fluff and spread gently.
Add another layer of graham crackers, then the remaining pudding mixture.
Spread reserved whipped cream over the top, scatter mini marshmallows, and drizzle with chocolate syrup.
Cover and refrigerate at least 6 hours or overnight before serving.