Preheat the oven to 350°F. Lightly spray a 9 x 9-inch baking pan with nonstick spray, then line it with parchment paper.
Add the cake mix, melted butter, eggs, and vanilla extract to a large mixing bowl. Mix until everything comes together into a thick cookie dough, about 1 to 2 minutes.
Fold in the chocolate chips.
Transfer the dough to the prepared baking pan and press it into an even layer. If the dough is sticky, lightly spray your hands or a rubber spatula with nonstick spray to make it easier to press into the pan.
Bake for 30 to 35 minutes, until the top is golden brown and the bars are set.
Allow the bars to cool in the pan for at least 15 minutes. Use the parchment paper to lift them out of the pan, then cut into 9 bars.
Notes
The dough should be thick and much denser than regular cake batter.
Be careful not to overbake the cookie bars, which can make them dry.
For an easy variation, substitute yellow or chocolate cake mix for the white cake mix.
Add 1/2 cup chopped walnuts, pecans, almonds, or pistachios along with the chocolate chips if desired.
Store cooled cookie bars in an airtight container at room temperature for 3 to 4 days.
To freeze, wrap the bars well and place them in a freezer-safe airtight container or freezer bag for up to 3 months. Thaw at room temperature before serving.