1. Preheat oven to 375 degrees F. Lightly coat an 8-inch-square nonstick baking pan with cooking spray. In a small bowl, whisk together egg, egg white, yogurt, and vanilla extract; set aside.
2. In a small saucepan, combine sugar, oil, salt, and 3 tablespoons water, stirring constantly for 3 minutes on medium-high heat. Remove from heat; stir in cocoa and chocolate until chocolate melts, then add egg mixture. Add all flour, stirring until batter is smooth.
3. Pour batter into baking pan. Bake 25 minutes, or until top is firm. Cool completely in pan on rack. Cut into squares.