Step 1: Preheat and Prepare Pan
Preheat oven to 350°F (175°C). Line an 8×8-inch square baking pan with parchment paper, leaving overhang on two sides for easy removal. Grease the parchment lightly with butter or cooking spray. Set aside.

Step 2: Melt Chocolate and Butter
In a medium saucepan over low heat, melt the butter and chopped chocolate together, stirring constantly until smooth and completely melted. Remove from heat and let cool for 5 minutes. (Alternatively, melt in microwave in 30-second intervals, stirring between each.)

Step 3: Mix in Sugars
Add granulated sugar and brown sugar to the chocolate mixture. Whisk vigorously for about 1 minute until well combined and slightly glossy. The mixture should be smooth.

Step 4: Add Eggs and Vanilla
Add eggs one at a time, whisking well after each addition. The batter should become thick and glossy. Add vanilla extract and whisk until combined.

Step 5: Add Dry Ingredients
In a small bowl, whisk together flour, cocoa powder, baking powder, and salt. Add the dry ingredients to the chocolate mixture. Using a rubber spatula, fold gently just until no flour streaks remain. Don’t overmix! The batter should be thick and fudgy-looking.

Step 6: Prepare Tahini Swirl
In a small bowl, whisk together tahini, powdered sugar, melted butter, and a pinch of salt until smooth and pourable. If mixture is too thick, add 1-2 teaspoons of water to thin slightly.

Step 7: Layer and Swirl
Pour brownie batter into prepared pan, spreading evenly with a spatula. Drop spoonfuls of the tahini mixture over the top of the brownie batter (about 8-10 dollops scattered across the surface). Use a butter knife or toothpick to swirl the tahini through the chocolate batter, creating a marble pattern. Don’t over-swirl – you want distinct ribbons, not a muddy mixture. 3-4 swirls through each section is perfect.

Step 8: Bake
Bake for 30-35 minutes. The brownies are done when the edges look set and the center is just barely set with a slight jiggle. A toothpick inserted near the center should come out with moist crumbs (not wet batter, but not completely clean). Don’t overbake! For 8×8 pan, check at 30 minutes. For 9×9 pan, start checking at 28 minutes.

Step 9: Cool and Slice
Remove from oven and place pan on a wire rack. Let cool completely in the pan for at least 1 hour (or overnight for cleanest cuts). Once cool, use the parchment overhang to lift brownies out of pan. Cut into 16 squares with a sharp knife, wiping the blade clean between cuts for the neatest slices.

Step 10: Optional Finishing
Before serving, sprinkle with flaky sea salt and toasted sesame seeds if desired for beautiful presentation and extra flavor.