I use hard boiled egg yokes and one raw yoke for my potato salad.
3 hard-boiled egg yolks, still warm
1 raw egg yolk
1 tablespoon white vinegar
1 cup vegetable oil
In a large mixing bowl, mash the cooked yolks very well with a fork. Add the raw yolk and vinegar and beat with a metal whisk or large spoon for a few seconds until very creamy. Gradually add the oil in a thin steady stream, whisking constantly or stirring briskly and making sure all the oil is incorporated before adding more. Refrigerate for at least 30 minutes before serving.