Preheat the oven to 350°F (175°C) and grease a 9×13 inch baking pan.
Combine the cake mix, melted butter, eggs, and vanilla extract in a large bowl. Mix until well combined and a thick dough forms.
Fold in the mix-ins of your choice, such as chocolate chips, M&M’s, or nuts.
Press the dough into the prepared baking pan, spreading it out evenly.
Bake for 20-25 minutes until the edges are golden brown and a toothpick inserted into the centre comes clean.
Allow the cookie bars to cool completely in the pan before cutting into squares or bars.
Serve and enjoy your delicious cake mix cookie bars!