Adjust the oven rack to the lowest position and preheat oven to 350 degrees. Make a foil sling for a 9-inch by 13-inch baking pan. The first sheet should be 13 inches wide and the second sheets should be 9 inches wide. Lay sheets of foil in pan perpendicular to each other with extra foil hanging over the edges of the pan. Push foil into corners and up the sides of the pan, smooth foil flush to pan. Coat foil with nonstick spray.
Prepare the brownie mix according to the package directions and pour into the prepared pan. Bake until top of brownie is shiny and center is almost set, about 15 minutes.
Meanwhile, in a standing mixer fitted with the paddle attachment, beat the cream cheese, sugar and vanilla on medium speed until combined. Beat in sour cream. Add the eggs, one at a time, mixing on low speed until blended. Pour the mixture over the partially baked brownie, filling the mixture to almost the top of the pan.
Bake until center is almost set, about 40 minutes. Remove from oven and run a knife or metal spatula around the rim of the baking dish to loosen cake. Cool and then refrigerate 4 hours or overnight. Allow to stand 30 minutes at room temperature prior to serving. Remove from the pan using the foil as a handle.
Melt the chocolate chips in the microwave in 30-second increments at 50% power until melted. Drizzle over the bars and let stand until chocolate is firm.
Brownie mix: Take note of the package instructions and round up all of the ingredients (such as oil, eggs, or water) you'll need to prepare them according to the manufacturer's recipe. You can also make brownies from scratch if you prefer.
Sour cream: This adds the signature cheesecake tang to the top layer.
Chocolate chips: Choose semi-sweet, milk, or dark based on what you have handy or prefer.
Yield: This recipe makes 24 bars (you can cut them bigger, but they are rich). Slice the 9-inch by 13-inch pan into 6 rows one direction and 4 rows the other direction.
Storage: Store leftovers in the refrigerator for up to 4 days.
Make ahead: Bake and chill the Cheesecake Brownies up to 1 day in advance.
Topping: Skip the chocolate drizzle if you don't want to (or don't have time for) that quick extra step.
Marble swirl topping: Add small dollops of fruit jam to the cheesecake topping prior to baking for the second round, then use a knife or skewer to gently draw alternating lines through the cream to create a marbled look.