01 - Preheat oven to 350°F. Line an 8-inch square baking pan with parchment paper, leaving an overhang for easy removal.
02 - Whisk together flour, cocoa powder, baking powder, and salt in a medium bowl. Set aside.
03 - Combine melted butter, sugar, and black sesame paste in a large mixing bowl. Whisk until smooth and glossy.
04 - Add eggs one at a time, whisking well after each addition. Stir in vanilla extract.
05 - Fold dry ingredients into the wet mixture using a spatula, mixing until just combined.
06 - Gently fold in the chopped dark chocolate.
07 - Pour batter into the prepared pan and smooth the top. Sprinkle black sesame seeds over the surface if using.
08 - Bake for 28-32 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
09 - Let cool completely in the pan before lifting out and slicing into squares.