In a stand mixer or in a large bowl using a hand mixer, cream together cold butter, brown sugar, and granulated sugar for 2-4 minutes or until fluffy.
Gently stir in eggs and vanilla extract, then mix until well combined (about 1 minute).Add the tahini and continue mixing.
In a large bowl combine the bread flour, all-purpose flour, cornstarch, baking soda, and salt.Gradually add the dry ingredients to the wet ingredients in ½ cup increments (mixing after each addition until just barely combined).
Stir in the chocolate chips until combined.
Scoop the dough into roughly shaped balls, 2-3 heaping tablespoons at a time, onto a baking sheet.
Bake for 9-10 minutes, until golden brown.The cookies will be very gooey when first out of the oven, so allow them to cool somewhat before moving from the baking sheet.