Preheat the oven to 375 degrees F (190 degrees C).
Grease a 9x13-inch pan or two 8x8-inch pans.
Combine 1/2 of the Yellow Cake Mix (1 package), Water (1/4 cup), Egg (2), Butter (1/4 cup), and Brown Sugar (1/4 cup). Mix well.
Blend in remaining cake mix. Then stir in Chocolate (12 ounce) and Walnut (1/2 cup).
Spread batter evenly in greased pan(s) and bake for 30 to 35 minutes.
Let cool and slice into bars or squares. Enjoy!