- Place the potatoes in a large pot of salted cold water and cook until al dente, about 20 minutes. Cool under cold running water and drain well. Cut into halves or quarters. Let cool and set aside.
- In a double boiler, beat the egg yolk and vinegar until creamy, about 4 minutes. Remove from the heat and, while whisking, drizzle in the butter. Add the sugar and salt. Combine and set aside at room temperature.
- In a large bowl, combine the mayonnaise, green onions, celery, and potatoes. In another bowl, whip the cream. Fold gently into the cooled sauce, then add to the potato salad. Season with salt and pepper.
Whether to bring them to a picnic, to eat outside, or to serve on a buffet, dishes containing cream or other dairy products, eggs, mayonnaise, meat, fish, or poultry must be kept no longer than 2 hours at room temperature. This standing time is even shorter when the food is exposed to hot summer days. Do not forget to place an ice pack in your picnic basket.