1) Whisk the vinegar, salt, and pepper in a small bowl. Gradually whisk in the oil, starting with a few drops and then adding the rest in a steady stream, to make a smooth dressing.
2) Shear off the corn kernels with a sharp knife over a bowl. Toss in the tomatoes, spring onions, and mozzarella. Pour the vinaigrette over the salad and toss to coat. Cover and let sit for 15 minutes or up to 2 hours. Before serving, tear the basil over the salad and toss.