Spread 1 teaspoon butter over each slice bread. Turn bread over; spread 1 teaspoon mustard over each slice bread. Layer ham and cheese over one slice of bread; close sandwich mustard sides in.
Beat egg white in a pie plate or shallow bowl; beat in milk until well blended. Heat a nonstick skillet over medium heat until hot.
Dip sandwich in egg mixture to coat both sides lightly. Place sandwich in hot skillet; cook 3 to 4 minutes per side or until golden brown and cheese is melted.
Cut sandwich diagonally in half; transfer to serving plate. Place powdered sugar in a strainer; shake over sandwich.
Serve with preserves.