Boil the potatoes as you would normally, drain them, and immediately pour the vinegar over them. Let them cool off for 5 to 10 minutes.
Whip the cream gently just to thicken it. Fold in the mustard, shallot, and tarragon. Season with salt and pepper.
When the potatoes have cooled, toss them gently with the cream. Add more salt and pepper to taste.
