How to Make It
In a large heavy skillet, heat the butter over medium high heat until hot. Stir in the onion and garlic with 1 teaspoon salt and 1/2 teaspoon pepper, and cook, stirring occasionally, until the onion is golden, about 8 minutes. Transfer the onion mixture to a blender and puree with the mango, ketchup, Worcestershire, vinegar, mustard, coriander, cumin, cayenne. Season with salt and pepper to taste.
Reserve 1/2 cup of the BBQ sauce, then toss the remaining sauce with the chicken and let marinate at room temperature 30 minutes.
Preheat the grill.
Grill the chicken over indirect heat, basting occasionally with the reserved BBQ sauce, until charred in places and cooked through, 35 to 45 minutes.