1. Preheat the grill to high heat.
2. Place butter and miso in a microwave-safe dish. Microwave at HIGH 30 seconds or until butter melts. Stir to combine.
3. Coat lobster tails with cooking spray. Place tails, flesh-side down, on grill rack coated with cooking spray; grill 4 minutes. Turn tails over; brush with butter mixture. Grill 3 minutes or until done, brushing occasionally with butter mixture. Serve with remaining butter mixture.