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1
In a kettle cover potatoes with salted cold water by 2 inches and simmer until just tender, about 25 minutes.
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2
Drain potatoes in a colander.
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3
When potatoes are just cool enough to handle, halve larger ones and in a large bowl toss potatoes with 2 tablespoons vinegar.
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4
In a small bowl whisk together mustard, remaining 3 tablespoons vinegar, and salt and pepper to taste.
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5
Add oil in a slow stream, whisking until emulsified.
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6
Dressing may be made 1 day ahead and chilled, covered.
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7
Bring dressing to room temperature before using.
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8
Trim beans and, working over a bowl, cut corn kernels from cobs.
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9
Have ready a bowl of ice and cold water.
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10
In a large saucepan of boiling salted water cook beans until crisp-tender, about 1 minute for haricots verts or about 5 minutes for regular green beans, and transfer with a slotted spoon to ice water to stop cooking.
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11
Drain beans well and add to potatoes.
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12
Return water in pan to a boil and blanch corn 30 seconds, or until crisp-tender.
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13
Drain corn in a sieve and rinse under cold water to stop cooking.
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14
Drain corn well and add to salad.
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15
Salad may be prepared up to this point 1 day ahead and chilled, covered.
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16
Bring salad to room temperature before proceeding.
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17
Gently toss salad with dressing and salt and pepper to taste until combined well.
