You can substitute an equal amount of celery salt for the celery seed, but if you do, eliminate the (unflavored) salt from the dressing. Spectrum naturals Organic Apple Cider Vinegar, Unfiltered, is the test kitchen's taste-test winner. Make sure you use sturdy Yukon Golds here; fluffy russets will fall apart in the salad.
3 pounds Yukon Gold potatoes, peeled and cut into 3/4-inch chunks
Salt and pepper
1/3 cup cider vinegar
1/4 cup sugar
2 tablespoons yellow mustard
4 large, hard-boiled eggs, peeled
1/2 teaspoon celery seed
3/4 cup sour cream
1 celery rib, chopped fine
Bring potatoes, 1 tablespoon salt and enough water to cover by 1 inch to boil in Dutch oven over high heat. Reduce heat to medium and simmer until potatoes are just tender, about 10 minutes.
While potatoes simmer, microwave the vinegar and sugar in small bowl until sugar dissolves, about 30 seconds. Process vinegar mixture, mustard, 1 hard-cooked egg yolk (reserve white), celery seed, and 1/2 teaspoon salt in food processor until smooth, about 10 seconds. Transfer to medium bowl.
Drain potatoes thoroughly, then transfer to large bowl. Drizzle 2 tablespoons dressing over hot potatoes and, using rubber spatula, gently toss until evenly coated. Refrigerate until cooled, at least 30 minutes, stirring gently once to redistribute dressing.
Whisk sour cream into remaining dressing. Add remaining hard-cooked eggs and egg white to dressing and, using potato masher, mash until only small pieces remain. Add dressing and celery to cooled potatoes. Cover and refrigerate until chilled, about 30 minutes. Season with salt and pepper to taste. Serve. (Salad can be refriegerated for up to 2 days.)