Preheat the oven to 175°C (160°C fan). Line 20 cm square baking tin with baking paper, with plenty of overhang.
Place the unsalted butter and 100g of the sugar in a small saucepan over a medium heat and allow both to melt completely, swirling the pan from time to time. Set aside.
Place the eggs and the remaining 200g of sugar into a bowl of an electric mixer and whish until pale and fluffy, about 2 minutes. Add the melted butter and sugar mixture and whisk again to combine.
In a medium bowl, combine together the dry ingredients: flour, cocoa powder and salt. Add them to the wet ingredients and fold, using a spoon or spatula, until combined. Add chopped chocolate, followed by the tahini, and gently fold together.
Pour the brownie batter into prepared baking tin and sprinkle the chopped walnuts (if using) over the top. Bake the brownies for 35-38 minutes, or until the top of the brownies is set and shiny, with few cracks in the surface. Toothpick inserted in the middle should come out with some crumb still attached to it.
Allow the brownies to cool and set in the tin for at least an hour, or ideally, until completely cool.
For the drizzle, mix melted chocolate and tahini in a small bowl, then drizzle it on top of the cooled brownies. Slice the brownies into 9 slices and enjoy!
Store any leftovers in the airtight container at room temperature for up to 4 days, or in the fridge, for up to 6 days.
Freeze the brownies for up to three months. Thaw in the fridge overnight, then bring to room temperature before serving.
You can reheat the brownies in the oven or in the microwave. Set the oven temperature to 160 C and place the brownies inside for 5 minutes, or place them in the microwave for 15-20 seconds.