Preheat your oven to 350°F (175°C). Grease a 9×13 inch baking pan or line it with parchment paper.
In a large mixing bowl, combine the yellow cake mix, eggs, melted butter, and vanilla extract. Stir until well combined and a thick dough forms.
Fold in the semi-sweet chocolate chips evenly throughout the dough.
Spread the dough evenly into the prepared baking pan, pressing it down gently with a spatula to create an even layer.
Bake in the preheated oven for 18 to 20 minutes, or until the edges are golden brown and a toothpick inserted in the center comes out with a few moist crumbs.
Remove from the oven and allow the bars to cool completely in the pan on a wire rack before cutting into 16 squares.
Serve and enjoy your chewy cake mix cookie bars!