Preheat oven to 425 degrees.
In a medium bowl, combine baking mix, milk and egg. Whisk just until the large lumps disappear. (The batter should have small lumps.) Pour batter into a 9x9-inch baking pan coated with cooking spray; sprinkle blueberries or chocolate chips over batter.
Bake for 13 to 15 minutes or until the top is light golden brown. Sprinkle with powdered sugar if desired. Slice and serve with maple syrup.