Preheat your oven to 350°F (175°C) and prepare an 8x8-inch baking pan by lining it with parchment paper.

In a medium saucepan, melt the butter over low heat. Once melted, stir in the sugar until combined.

Whisk in the eggs, one at a time, followed by the vanilla extract.

In a separate bowl, combine the flour, cocoa powder, and salt. Gradually add this mixture to the melted butter mixture and stir until just combined.

In another mixing bowl, combine the softened cream cheese and peanut butter until smooth. Add in the sugar, egg, and vanilla extract, blending until well incorporated.

Spread half of the brownie batter into the bottom of the prepared pan.

Dollop spoonfuls of the peanut butter cheesecake mixture on top and carefully spread it.

Drop the remaining brownie batter over the top and swirl together lightly with a knife.

Bake for 30-35 minutes, or until a toothpick comes out with moist crumbs. Let cool in the pan for about 15 minutes before transferring to a wire rack.

Once cool enough, cut into squares and serve.