1. Rinse the shrimp under cold running water, then blot them dry with paper towels. Soak two to three skewers in water.
2. Place the chile powders, garlic, coriander, cumin, and pepper in a mixing bowl and whisk to combine. Add the shrimp and toss to coat with the spice rub. Stir in the olive oil. Let the shrimp marinate in the refrigerator, covered, for 30 minutes to 1 hour.
3. Preheat barbecue to high. Skewer shrimp. Cook the shrimp for 2 to 4 minutes until firm.
4. With the barbecue still on high, coat the ears of corn with olive oil and place directly on grill. Rotate slowly roasting each side of the corn until slightly blackened. Roast for 15-20 minutes.
5. Cool corn on the cob. Using a large knife, carefully remove corn from the cob.
6. Gently combine roasted corn, mangoes and avocado in a large bowl with the juice of two limes. Season with salt and pepper to taste.
7. Spoon the avocado, mango, corn cocktail into 4 large glasses or serving bowls. Drape 5 of the hot shrimp over the edge of each glass or bowl and serve at once. Or for a cold shrimp cocktail, let the cooked shrimp cool to room temperature. Refrigerate the shrimp, covered, until they are chilled before serving them with the cocktail. The cooked shrimp can be refrigerated for up to 2 days.

Serves 4.