Heat the oven to 350°F (175°C). Grease a 13 x 9-inch baking pan or line it with parchment, leaving a little overhang to make lifting the cooled brownies easier.

Prepare the fudge brownie mix according to the directions on the package, adding the eggs, vegetable oil, water, or other listed ingredients exactly as instructed. Stir just until the batter is combined.

Reserve about 1/2 cup of the brownie batter. Spread the remaining batter evenly in the prepared pan, taking it all the way into the corners.

Beat the softened cream cheese, creamy peanut butter, sugar, egg, and vanilla extract until smooth. Scrape the bowl as needed so the mixture is evenly blended and free of large cream cheese lumps.

Spread the peanut butter cheesecake mixture over the brownie batter. Work gently so the layers remain distinct instead of mixing together.

Drop spoonfuls of the reserved brownie batter over the cheesecake layer. Swirl gently with a knife in a few sweeping motions, taking care not to over-swirl.

Bake for 35–40 minutes, until the edges are set and the center is only slightly soft. A toothpick inserted into a brownie portion should show moist crumbs rather than wet batter.

Cool the brownies completely in the pan for at least 1 hour. For cleaner slices, refrigerate the cooled brownies for 1–2 hours, then cut with a sharp knife wiped clean between slices.