Heat the oven to 175 C / 350 F. Line an 8x8 inch baking pan with parchment paper, leaving a 2-inch overhang on two sides.

Make the sesame swirl: beat the cream cheese with a hand mixer until smooth, about 1 minute. Add the black sesame paste, powdered sugar, egg yolk, and vanilla. Beat until fully combined and no lumps remain. Set aside.

Set a heatproof bowl over a pot of barely simmering water. Add the chopped chocolate and butter. Stir occasionally until fully melted and smooth, about 4 minutes. Remove from heat.

Whisk the granulated sugar into the warm chocolate mixture until combined. Add the 2 whole eggs and 1 yolk one at a time, whisking after each addition until the batter looks glossy and slightly thickened.

Stir in the vanilla extract. Sift the flour, cocoa powder, and salt directly into the bowl. Fold with a rubber spatula until just combined and no dry streaks remain.

Pour the brownie batter into the prepared pan and spread evenly with the spatula.

Drop the black sesame mixture in 8 to 10 small spoonfuls across the surface of the brownie batter.

Drag a wooden skewer or toothpick through the spoonfuls in long S-curves and figure-eights to create a marbled pattern. Do not over-swirl or the pattern will muddy.

Bake for 23 to 25 minutes, until the edges are set and pulling away from the pan and the center still has a slight wobble when gently shaken.

Remove from the oven and let cool completely in the pan on a wire rack, then refrigerate for at least 2 hours before slicing.

Lift the brownie slab out of the pan using the parchment overhang. Place on a cutting board.

Run a sharp knife under hot water, wipe dry, then cut into 16 squares (4x4 grid). Wipe the knife between cuts for clean edges.