How to Make It
Whisk together buttermilk, mayonnaise, white wine vinegar, tarragon, garlic, 1⁄4 teaspoon of the salt, and 1⁄8 teaspoon of the pepper in a small bowl. Slowly drizzle oil into mixture, whisking constantly, until combined.
Measure 2 tablespoons of buttermilk dressing into a medium bowl. (Reserve the remaining dressing for serving.) Add scallops to bowl, and toss to coat. Marinate at room temperature 10 minutes. Thread scallops onto skewers (about 5 per skewer). Discard marinade.
Preheat a gas grill to high (450°F to 500°F). Place mango planks on oiled grates. Grill, uncovered, until slightly charred, 1 to 2 minutes per side. Remove mango from grill; cut into 1⁄2-inch cubes. Set aside. Re-oil grates; place scallop skewers on grill. Grill, uncovered, until charred, about 1 minute per side.
Line a platter with lettuce. Top with mango, avocado, radish, and scallop skewers. Sprinkle salad with remaining 1⁄4 teaspoon salt and 1⁄8 teaspoon pepper. Drizzle with reserved dressing, and serve immediately.