Preheat the oven to 350ºF. Line the inside of an 8” x 8” square or round baking dish with parchment. Lightly grease the paper and the pan with butter or cooking spray.

Melt the butter and chocolate together in the microwave, or in a heat-proof bowl placed over a small pot of boiling water. Allow the chocolate to cool slightly.

In a large bowl, mix together the cooled melted chocolate, sugar, brown sugar, and salt. Add the eggs, one at a time, mixing each one until fully incorporated.

Sift together the cocoa powder and potato starch into the chocolate mixture. Using a wooden spoon, beat the batter vigorously for 1-2 minutes, until the batter is no longer grainy, starts to look very shiny and smooth, and pulls away from the sides of the bowl. Transfer the batter into the greased baking dish.

Dollop the top with spoonfuls of tahini, then using a skewer or knife, swirl the tahini in one direction through the batter. Swirl the tahini in the opposite direction to create a pattern.

Bake for 22-25 minutes. For a fudgier brownie, take the brownies out when they are just set and firm at the edges. If you like a firmer brownie, you can cook until the center looks set.

Immediately after taking the brownies out of the oven, sprinkle with flake salt, if using. Let cool completely before removing from the pan and slicing.