Preheat oven to 350 degrees F. Butter and line an 8x8 cake tin with parchment paper.
In a large bowl, mix together melted butter, sugar, and salt until well combined. Add in three eggs, and vanilla extract and stir until just mixed.
Sift in flour and cocoa powder, and fold until no streaks of flour can be seen. Pour into prepared tin, and smooth out into an even layer.
In a small bowl whisk together remaining egg, tahini, and brown sugar until smooth. Add dollops over the surface of the brownie, and swirl using a toothpick to create beautiful designs.
Bake for 35 minutes. Cool in pan on a wire rack for a minimum of 15 minutes before slicing and serving. Enjoy!
