2 large egg yolks
1 teaspoon salt
2-3 teaspoons lemon juice
200ml olive oil

Have all the ingredients at room temperature.  Put the egg yolks into a small basin, add the salt and 1 teaspoon of the lemon juice and mix thoroughly.  Add the oil, drop by drop, stirring constantly, until the sauce becomes thick and shiny.  Add the rest of the oil in a thin stream, stirring constantly.  Add lemon juice to taste.