1. In small saucepan, stir together egg yolks, vinegar, water, mustard, salt and pepper until thoroughly blended.
2. Cook over VERY LOW heat, stirring constantly until mixture bubbles in just 1 or 2 places. Remove from heat. Let stand 4 minutes.
3. Pour into blender container. Cover and blend at high speed. While blending, very slowly add oil. Blend until thick and smooth. Occasionally, turn off blender and scrape down sides of container with rubber spatula if necessary.
4. Cover and chill if not using immediately.