In a large bowl, combine dry cake mix, brown sugar and cocoa. Stir in eggs, oil, vanilla and water. Mix in chocolate chips and pecans.
Spray a 13"x9" baking pan with non-stick vegetable spray; spread in batter.
Bake at 350 degrees for 18 to 20 minutes. Cool and spread with whipped topping and additional pecans, if using. Cut into bars.