Melt butter in a saucepan. Add one egg, mix, add sugar, mustard and vinegar. Stir until smooth. Return to fire and stir until it thickens, add sufficient corned beef liquid to make it the desired consistency. Stir until it boils, simmer for 3 minutes, then pour a little, very slowly over the beaten yolk of an egg. Return to saucepan and stir until it thickens, not boils.