3 pounds Yukon Gold potatoes, peeled and cut into 3/4-inch chunks
Salt and pepper
1/3 cup cider vinegar (see note)
1/4 cup sugar
2 tablespoons yellow mustard
4 large hard boiled eggs, peeled
1/2 teaspoon celery seed (see note)
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 large pot over high heat. Reduce heat to medium and simmer until potatoes are just tender, about 10 minutes.
While potatoes simmer, microwave vinegar and sugar in small bowl until sugar dissolves, about 30 seconds. Process vinegar mixture, mustard, 1 hard boiled egg yolk (reserve white), celery seed, and ½ teaspoon salt in food processor until smooth. 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 boiled 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. Serve. (Salad can be refrigerated in airtight container for 2 days.)
You can substitute an equal amount of celery salt for the celery seed, but if you do, eliminate the (unflavored) salt from the dressing. Maille Apple Cider Vinegar is the test kitchen’s taste-test winner. Make sure to use sturdy Yukon Golds here, fluffy russets will fall apart in the salad.