In a small bowl beat the egg yolks until thick with a little salt, pepper, half the vinegar, and the mustard. Add the oil, drop by drop, whisking constantly. After adding 2 tablespoons of the oil, the mixture should be quite thick. Add the remaining oil more quickly, 1 tablespoon at a time, or pour from a jug in a very slow stream, whisking constantly. Stir in the remaining vinegar and season with salt and pepper.
