Preheat oven to 350°F (175°C). Line a 9×9-inch baking pan with parchment paper, leaving some overhang for easy removal.
In a large mixing bowl, whisk together the melted butter and sugar until glossy. Beat in the eggs one at a time, then add the vanilla extract.
Sift in the cocoa powder, flour, baking powder, and salt. Fold gently with a spatula until just combined. Do not overmix.
In a separate bowl, beat the cream cheese, sugar, and heavy cream until smooth. Add eggs one at a time, beating well after each addition. Fold in the peanut butter powder until incorporated.
Spread about two-thirds of the brownie batter evenly into the prepared pan.
Pour the cheesecake mixture over the brownie base, spreading evenly.
Dollop the remaining brownie batter on top, then use a butter knife to gently swirl the layers together for a marbled effect.
Bake for 50–55 minutes, or until the center is just set and a toothpick inserted comes out with a few moist crumbs.
Cool completely in the pan before slicing into bars.