Preheat oven to 350ºF. Spray 9 x 13-inch Cake Pan with vegetable pan spray; set aside. Place butter and chocolate in 1-quart microwave-safe bowl. Microwave on 50% power 2-3 minutes, stirring every 30 seconds until chocolate is melted. Stir in sugar. Add eggs and vanilla extract; mix well. Stir in flour and salt; mix until just combined.