Preheat the oven to 350°F
Grease or line an 8×8-inch or 9×9-inch baking pan
In a bowl, combine 1 box of cake mix, 2 eggs, and 1/2 cup melted butter or oil
Add 1 to 2 tablespoons water if the batter seems too thick
Mix until fully combined
Fold in chocolate chips, nuts, or other mix-ins if desired
Spread the batter evenly in the prepared pan
Bake for 20 to 30 minutes, until the center is set
Let cool completely before cutting into squares