1. Preheat your oven to 350°F (175°C). Line an 8×8-inch or 9×9-inch square baking pan with parchment paper, leaving an overhang on the sides for easy removal.

2. Make the Brownie Base: In a medium microwave-safe bowl, combine the chopped chocolate and butter. Microwave in 30-second intervals, stirring in between, until completely melted and smooth. (Alternatively, use a double boiler). Set aside to cool slightly.

3. In a large mixing bowl, whisk together the granulated sugar, brown sugar, eggs, and vanilla extract until the mixture is pale and slightly thickened (about 2 minutes).

4. Pour the melted chocolate mixture into the egg mixture and whisk until glossy and combined.

5. Sift the cocoa powder, cornstarch, and salt directly into the bowl. Fold gently with a spatula just until no dry streaks remain. The batter will be thick.

6. Pour about ¾ of the brownie batter into the prepared baking pan, spreading it into an even layer. Reserve the rest.

7. Make the Cheesecake Swirl: In a medium bowl, beat the softened cream cheese, sugar, egg yolk, and vanilla with a hand mixer until smooth and creamy.

8. Dollop spoonfuls of the cream cheese mixture over the brownie layer in the pan.

9. Dollop the remaining brownie batter in the gaps between the cream cheese.

10. Use a butter knife or toothpick to gently swirl the batters together to create a marbled effect. (Don’t over-swirl, or the colors will just mix into a light brown).

11. Bake for 30–35 minutes. The edges should be set and slightly puffed, and the center should look set but still slightly soft (it will firm up as it cools). A toothpick inserted into the brownie part should come out with moist crumbs, not wet batter.

12. Cool completely in the pan on a wire rack. For the cleanest slices and best texture, refrigerate the brownies for at least 2 hours before cutting.