Preheat the oven to 350°F (175°C) and line a 9x9-inch baking pan with parchment paper.
In a mixing bowl, whisk together the melted butter and granulated sugar until smooth.
Add the eggs and vanilla extract, mixing until fully combined.
Sift in the cocoa powder, salt, and baking powder, stirring to incorporate.
Fold in the flour until just combined, being careful not to overmix.
In a separate bowl, beat the softened cream cheese, sugar, egg, and peanut butter until smooth and creamy.
Pour the brownie batter into the prepared baking pan and spread evenly.
Drop spoonfuls of the peanut butter cream cheese mixture onto the brownie batter.
Use a toothpick or knife to gently swirl the peanut butter mixture into the brownie batter.
Bake for 28-32 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Allow the brownies to cool in the pan for at least 20 minutes before slicing and serving.