In a small saucepan over medium heat, whisk together sour cream, heavy cream, yolks, mustard, vinegar, sugar, and 1/2 teaspoon salt. Cook, whisking constantly, until sauce thickens and coats back of spoon, 4 to 5 minutes.
Immediately transfer to serving bowl, stir in chives, and season with salt and pepper to taste. Serve warm or at room temperature. Makes about 3 cups.