In a medium saucepan cook potatoes, covered, in enough boiling salted water to cover for 5 minutes or until just tender. Drain and cool. Cut corn from cobs. On a large serving platter arrange potatoes and tomatoes. Sprinkle with corn and the 1/4 cup basil.
For dressing, in a screw-top jar combine oil, vinegar, shallot, mustard, sugar, and salt and pepper to taste. Cover and shake well. Pour dressing over potato mixture. Sprinkle salad with feta cheese and basil leaves.