Heat your oven to 350 degrees F. Line an 8x8 inch baking pan with parchment paper, leaving a couple inches of overhang on two sides so you can lift the brownies out cleanly. Lightly grease the parchment.

In a medium heatproof bowl set over a small saucepan of barely simmering water, melt the butter and dark chocolate together, stirring occasionally until completely smooth. Remove the bowl from the heat and let it cool for 5 minutes. You can also do this in a microwave in 30-second bursts, stirring between each.

Whisk the granulated sugar into the melted chocolate mixture until combined. Add the eggs one at a time, whisking well after each. Stir in the vanilla. The batter should look glossy and slightly thick at this point.

Add the flour, cocoa powder, and fine sea salt to the bowl. Switch to a rubber spatula and fold everything together until just combined, meaning no dry streaks remain. Do not overmix or the brownies will be cakey instead of fudgy.

In a small bowl, stir the tahini and maple syrup together until smooth and pourable. If your tahini is very thick, stir in 1 teaspoon of warm water to loosen it slightly.

Pour the brownie batter into the prepared pan and spread it into an even layer. Drop spoonfuls of the tahini mixture across the surface, spacing them fairly evenly. Use a toothpick or the tip of a butter knife to drag through the tahini drops in long S-curves or figure-eights, creating a swirl pattern. Do not overwork it or the swirl will disappear into the batter.

Scatter a pinch of flaky sea salt over the top if you like. Bake for 28 to 32 minutes. The edges should pull slightly away from the pan sides and the center should look set but not dry. A toothpick inserted 1 inch from the edge should come out with just a few moist crumbs, not wet batter.

Let the brownies cool completely in the pan on a wire rack before lifting them out using the parchment overhang. Cut into 9 squares. The brownies slice cleanest when fully cooled.