Whisk together pudding mix and milk in a medium bowl. Whisk for 2 minutes until combined. Place in fridge for 5 minutes to chill and thicken.
Place 9 graham crackers squares in bottom of a 9x9-inch glass baking pan. (They may overlap.)
Add cool whip to pudding. Stir to combine. Spread half of the pudding mixture over the graham crackers in pan. Top with remaining graham crackers and spread the rest of pudding over those graham crackers.
Top with marshmallows and chocolate syrup. You can add some more crushed graham crackers over the top as well. Cover and chill dessert for at least 3 hours before serving.