In a mixing bowl, using an electric mixer, cream together the butter and sugar.
Add the water and egg.
Add the yellow cake mix and combine well.
Stir in the chocolate chips and chopped walnuts; if using.
Pour mixture into a 9 X 13 size pan that has been sprayed with non-stick cooking spray.
Bake in a 350 degree preheated oven for 30 to 35 minutes or until a knife inserted into the center comes out clean.
Let cool slightly before cutting in serving pieces.
Notes
Leftover bar cookies can be stored in an airtight container at room temperature for up to 3 days.You can freeze them in an airtight container for up to 3 months. Wrap each cookie individually in plastic wrap or wax paper. This will help to prevent them from sticking together and drying out.