Make Batter: In a bowl, whisk together flour, milk, egg, sugar, vanilla extract, baking powder, and salt until smooth. Let rest for 5 minutes.
Heat Oil: In a deep pan or fryer, heat oil to 350°F (175°C).
Coat Marshmallows: Dip each marshmallow into the batter, then roll in crushed graham crackers if using for extra crunch.
Fry: Carefully drop marshmallows into hot oil. Fry for about 30–45 seconds, turning once, until golden brown.
Drain & Serve: Remove with a slotted spoon and drain on paper towels. Dust with powdered sugar or drizzle with chocolate sauce before serving.