Preheat oven to 400° Lay sweetpotatoes in a single layer on a large, rimmed baking sheet. Drizzle with 1 tablespoon of the oil. Sprinkle with 1/4 teaspoon of the salt and 1/4 teaspoon of the pepper; toss. Bake 30 minutes tossing once until golden. Let cool.
In a large skillet, char ears of corn over high heat, about 5 to 10 minutes; let cool. In the same skillet, char radicchio wedges over medium-high heat. Let cool. Cut off the corn kernels with a sharp knife; set aside. Coarsely chop the radicchio; set aside.
In a small bowl combine buttermilk, yogurt, mayonnaise, lemon juice, mustard, honey, garlic, dill, chives, remaining 3/4 teaspoon salt, remaining 1/2 teaspoon pepper and remaining 2 tablespoons olive oil.
Toss shredded lettuce with red onion and 3 tablespoons of the dressing. Transfer to a large serving platter. Arrange sweetpotato, corn, radicchio, bacon, eggs, chicken, avocado, tomatoes, blue cheese and zucchini on top of lettuce. Serve dressing on the side.