Rub one hard-boiled egg-yolk through a sieve, put in a porcelain or enamel basin, and stir with a small amount of vegetable oil. Add one raw egg-yolk, keep on stirring with the vegetable oil, pouring it in drop by drop until thick. Salt, add the mustard, lemon juice or vinegar, and sugar. This mayonnaise must be used within 24 hours.