Combine vinegar, mustard, honey, ½ teaspoon salt and black pepper in a glass measuring cup. Stir to combine. Whisk in olive oil until thick and emulsified. Taste and adjust with more salt and pepper if needed.
Preheat oven to 400°F. Line a baking sheet with aluminum foil and set aside.
Cut the sourdough into 1-inch thick slices. Cut the slices into 1-inch thick strips then tear into bite-sized pieces.
Toss the bread cubes with melted butter and season with 1/2 tsp salt and 1/4 teaspoon black pepper. Spread out evenly on the prepared baking sheet and bake for 12- 15 minutes, or until golden brown and crunchy. Let cool.
Bring a medium pot of salted water to boil. Fill a bowl with ice water and set aside. Add the corn to the boiling water and cook for about 30 seconds. With a slotted spoon, remove the corn from the hot water and plunge into ice water to stop the cooking. Drain.
In a large bowl, combine cooled bread cubes, blanched corn, heirloom tomatoes, cherry tomatoes, cucumber, mozzarella and basil. Toss with the dressing, taste, and adjust seasoning if needed. Transfer to a large platter and serve.