Melt the butter in a bowl, using either a microwave-safe dish or gently warming it on the stovetop. Heat only until fully liquefied, taking care not to let it bubble vigorously or brown. Warm, melted butter helps the dry mix blend smoothly and makes the batter easier to spread later.
Whisk in the eggs one at a time, mixing until the yolks are fully broken and incorporated. The mixture should look glossy and uniform. Adding the eggs to warm—not hot—butter helps prevent accidental scrambling and ensures the batter remains soft and cohesive.
Stir in the yellow cake mix, folding gradually until no dry pockets remain. The batter will become thick and dense, which is expected for cookie bars. Mix until the ingredients are combined but do not overwork it, as excessive stirring can make the final texture slightly tougher.
Mix in the semi sweet chocolate chips, distributing them evenly throughout the thick batter. The dough should feel heavy and well-loaded with chips, ensuring that every bite contains melted chocolate once baked.
Press the batter into a greased pan, using a 9×13 inch dish for thinner, softer bars or a smaller baking pan if you prefer thicker, chewier pieces. Lightly greasing the pan ensures easy release after baking. Because the batter is thick, using a spatula or lightly oiled fingertips helps spread it into an even layer that bakes uniformly.
