In the bowl of a stand mixer, add in the butter, brown sugar, vanilla, salt and both eggs.
Use the paddle attachment to cream the mixture on medium speed for about 3 minutes until it becomes a few shades lighter. Scrape the sides and bottom of the bowl.
Dump in the bread flour, baking soda, and baking powder.
Use the mixer to mix on low until just combined. It's ok if some flour is still visible.
Melt the chocolate in a microwave safe bowl or on the stove in a double boiler.
Pour the tahini into the cookie dough and use a flat rubber spatula to mostly fold it into the batter, you want some still visible.
Pour the melted chocolate into the cookie dough and fold 5-6 times into the cookie dough. It should be visibly marbled, don't over mix.
I like big cookies so I used a quarter cup ice cream scoop but you could do any size you want, you'll simply need to adjust the baking time accordingly.
Line a small tray or baking sheet with parchment paper or a silicone baking mat and scoop the cookie dough on.
Once all the dough has been scooped, cover the cookies with plastic or a second baking sheet upside down.
Place in the fridge for at least 4 hours, ideally overnight or in the freezer for 2 hours.