Preheat the oven to 325 degrees F. Line an 8x8 inch brownie pan with parchment paper, allowing two sides to hang over the edges. Grease the pan with butter or baking spray.

In a large heatproof bowl set over a pot of simmering water, heat the bittersweet chocolate and butter together, stirring until smooth. Remove the pan from the heat.

Add the sugar and whisk vigorously until smooth. Whisk in the eggs, vanilla and salt. Sift in the flour and stir until smooth.

Pour the mixture into the prepared pan. Bake for 30-35 minutes or until a toothpick inserted into the center comes out clean. Allow the brownies to cool to room temperature.

In a stand mixer fitted with the paddle attachment, cream the butter on high speed until light and fluffy. Scrape down the sides. Add the tahini and mix on medium speed, scraping down the sides several times, until evenly combined with the butter.

Add the sugar and salt and turn the mixer on low speed until the dry ingredients have incorporated. Scrape down the sides and turn the speed up to medium high, allowing the frosting to mix for another minute until light and smooth.

Use the excess parchment paper to lift the brownies out of the pan and place them on a cutting board. Use a spatula to evenly frost the brownies. Cut into 16 squares and serve.