In the bowl of a stand mixer fitted with the paddle attachment (or you can use a hand mixer), beat the butter with the brown sugar, white sugar and tahini on medium speed for 3 minutes until the mixture becomes creamy and fluffy.
Add the eggs, vanilla extract, and lemon zest and mix for another 2 minutes.
In a separate medium bowl, mix the flour, baking soda and salt.
Add the flour mixture to the bowl and mix all ingredients on low until just combined.
Turn off the mixer and fold in the chopped chocolate using a wooden spoon (or spatula).
Cover the bowl and refrigerate for a minimum of 1 hour.
Preheat the oven to 350 degrees and prepare 2 baking sheets with parchment paper.
Using an ice cream scoop, or a big spoon, form the cookies into 2 inch round shapes and place them 3 inches apart on the baking sheet.
Bake for 10 to 15 minutes until golden on the edges and the middle still a bit soft.
Let the cookies cool on the baking sheet for 10 minutes before transferring to a wire rack (to avoid breaking the cookies). Transfer to a wire rack and allow to completely cool before serving.