In a heavy saucepan, melt chocolate over low heat. Stir in milk and vanilla; cook and stir until smooth. Making 1 s'more at a time, spread 1 tablespoon each of chocolate mixture on two whole graham crackers. Place 5 to 6 marshmallows on one cracker; gently press the other cracker on top. Repeat with remaining chocolate, crackers and marshmallows. Wrap with plastic wrap; store at room temperature.
