In a small bowl, combine the flour, milk, egg, vanilla extract, sugar, and salt to form a smooth batter.
Take two quarters of graham cracker and place a piece of chocolate and 1 to 2 marshmallows on one quarter. Top with another quarter of graham cracker to form a sandwich.
Dip the s'mores sandwich into the batter, ensuring it is fully coated.
Place the crushed graham crackers on a plate and coat the battered sandwich in the crushed graham crackers, pressing gently to adhere.
In a large skillet, heat about 1 inch of vegetable oil over medium heat until it's hot but not smoking.
Carefully place the coated sandwich into the hot oil and fry for 2-3 minutes on each side or until golden brown and crispy.
Using a slotted spoon, transfer the fried sandwich to a paper towel-lined plate to drain excess oil.
Dust with powdered sugar, if desired, and serve warm for a melty, gooey delight.