In a small heavy saucepan, combine the sugar, flour and salt. Stir in 3/4 cup cream and mustard until smooth. Cook and stir over medium-high heat until thickened and bubbly. Reduce heat; cook and stir 2 minutes longer.
Remove from the heat. Combine egg yolk and remaining cream. Stir a small amount of hot mixture into egg yolk mixture; return all to the pan, stirring constantly. Bring to a gentle boil; cook and stir 2 minutes longer. Remove from the heat. Gently stir in vinegar. Keep warm. Serve with pretzels.