In a small stainless bowl, beat together sour cream, mustard, and egg yolks.
Slowly add the olive oil, alternating with vinegar, stirring between each addition. A hand-crank eggbeater or wire whisk works well for this step.
Add salt and pepper to taste. Chill at least four hours before serving.
You can add anything to your sour cream mayo: herbs, spices, garlic, onions… anything! If the mayo turned out too thin for your liking, you can either add more sour cream to thicken it, or just use it as a delicious chilled salad dressing.