Preheat the oven to 350°F. Line an 8×8-inch baking pan with parchment paper, leaving some overhang for easy removal. Lightly spray the parchment and sides of the pan with nonstick cooking spray.
In a large mixing bowl, combine the 15.25-ounce box of chocolate cake mix, ⅓ cup vegetable oil, 2 large eggs, and 2 teaspoons vanilla extract. Stir until a thick, evenly combined batter forms.
Fold in the 1 cup semi-sweet chocolate chips until evenly distributed throughout the batter.
Spread the batter evenly into the prepared 8×8-inch pan. Use a spatula or lightly greased hands to press the thick batter into an even layer.
Bake for 18–22 minutes, or until the edges are lightly browned and the center is set. Be careful not to overbake, as the brownies will continue to firm up as they cool.
Remove the pan from the oven and let the brownies cool completely in the pan.
Once cooled, use the parchment paper to lift the brownies from the pan. Cut into 12 pieces and serve.
Notes
Store the leftover brownies in an airtight container at room temperature for up to 5-7 days.