Heat olive oil in a small saucepan over medium heat. Add onion and cook until soft. Add cascable and ancho powder and cook for 1 minute, let cool. Fold onion mixture into softened butter, add lime juice and season with salt and pepper to taste. Place in a bowl and refrigerate until firm.
Preheat grill. Brush lobster tails with oil and season with salt and pepper. Place meat side down and grill 3 to 4 minutes on each side or until cooked through. Remove from heat and top immediately with butter.