1. Preheat oven to 450°.
2. Line a rimmed baking sheet with aluminum foil. Whisk together the lime juice, 2 teaspoons olive oil, ancho chile powder, paprika, and 1/4 teaspoon salt in a small bowl. Place the corn on the prepared baking sheet, and brush with the lime mixture. Roast the corn for 30 minutes.
3. Remove corn from the oven. Adjust oven rack 4 inches from heat, and turn on broiler. Return corn to oven, and broil, turning, for 3 minutes or until browned in patches.
4. Whisk together buttermilk, grated Parmesan cheese, and remaining 2 teaspoons olive oil and 1/4 teaspoon salt in a small bowl. With a small, sharp knife, cut corn off cobs into a serving bowl; gently toss kernels with grape halves and mixed greens. Divide salad among 4 dinner plates. Garnish each with flowering herbs, if desired, and drizzle evenly with dressing; serve.