Step 1: Preheat your oven to 350°F (175°C). Lightly grease and flour a 9x13 inch baking pan, or line it with parchment paper leaving an overhang on the sides for easy removal.
Step 2: In a large bowl, whisk together the melted butter and eggs until well combined and slightly frothy.
Step 3: Add the entire box of yellow cake mix to the wet ingredients. If using, stir in the vanilla extract.
Step 4: Mix with a spoon or spatula until just combined. The dough will be thick. Do not overmix.
Step 5: Fold in the chocolate chips until evenly distributed throughout the dough.
Step 6: Press the cookie dough evenly into the prepared 9x13 inch baking pan. A rubber spatula or your lightly floured hands work best for this.
Step 7: Bake for 20-25 minutes, or until the edges are lightly golden brown and a toothpick inserted into the center comes out with moist crumbs (not wet batter). Be careful not to overbake for chewier bars.
Step 8: Remove from the oven and let cool completely in the pan on a wire rack before cutting into squares. This is crucial for clean slices.