Preheat your oven to 350°F (175°C). Line a baking pan with parchment paper, leaving some overhang for easy removal.

In a saucepan over low heat (or in the microwave), melt the butter and semi-sweet chocolate together, stirring until smooth. Remove from heat and allow to cool slightly.

In a large bowl, whisk together the melted chocolate mixture, granulated sugar, and brown sugar until smooth.

Add the eggs one at a time, whisking well after each addition.

Stir in the vanilla extract.

In a separate bowl, whisk together the flour, cocoa powder, and salt.

Gradually fold the dry ingredients into the wet ingredients using a spatula, mixing just until combined. Do not overmix.

Pour about ¾ of the brownie batter into the prepared pan and spread evenly.

In another bowl, beat the softened cream cheese until smooth using an electric mixer.

Add sugar, egg, and vanilla extract to the cream cheese and mix until creamy and lump-free.

Spoon the cheesecake mixture over the brownie layer in dollops.

Add the remaining brownie batter on top in spoonfuls.

Use a knife or toothpick to gently swirl the brownie and cheesecake layers together for a marbled effect.

Bake for 35–45 minutes, or until the edges are set and the center is slightly firm with a gentle jiggle.

Remove from the oven and allow the brownies to cool completely in the pan before slicing.