Preheat the oven to 350°F (175°C). Grease a 9×13-inch baking pan or line it with parchment, leaving an overhang for easy removal.

In a medium saucepan over low heat, melt the butter and chopped dark chocolate together. Stir constantly until completely smooth. Remove from heat.

Whisk in the granulated sugar and brown sugar until glossy. Add the eggs one at a time, whisking after each. Stir in 1 tsp vanilla extract.

Fold in the flour, cocoa powder, and salt. Mix until just combined — a few streaks are fine; don’t overwork the batter.

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

In a bowl, beat the softened cream cheese with powdered sugar and 1/2 tsp vanilla until silky and lump-free.

Dollop spoonfuls of the cream-cheese mixture over the brownie layer. Use a knife or skewer to gently swirl — aim for pretty marbling, not full blending.

Spoon the remaining brownie batter over the swirled layer and gently spread to cover.

Bake 25–30 minutes, or until a toothpick inserted near the center comes out mostly clean with a few moist crumbs.

Cool completely in the pan on a wire rack. Once cool, lift out using the parchment overhang and cut into bite-sized squares.