Preheat your oven to 350F. Line an 8-inch baking dish with parchment paper, or grease the dish well.
Add the cake mix, melted butter, eggs, and vanilla to a mixing bowl. Stir until well combined.
1 15.25 ounce chocolate cake mix, 1/2 cup unsalted butter, 2 whole large eggs, 1 teaspoon vanilla extract
Add the chocolate chips and stir until they are evenly distributed. The batter. will be quite thick.
1 cup semi-sweet chocolate chips
Spread the brownie batter into the prepared baking dish. Bake for 22-25 minutes, until the top is dry, the edges appear done, and a cake tester comes out mostly clean.
Remove the brownies from the oven and let cool completely before slicing.
Notes
Store leftovers in an airtight container on the counter for 3 days, or in the fridge for 1 week.
You can freeze these brownies for 3 months in a double layer of airtight plastic bags.
For best results, use room-temperature eggs.
Make sure there's still a few crumbs on the cake tester when you remove the brownies. If the tester comes out perfectly clean, the brownies are overdone.