In a large bowl, whisk the sugar, butter and tahini until creamy and smooth. Whisk in the egg until smooth. Fold in the flour, baking soda and chocolate and stir to form a dough, then rest in the fridge for 30 minutes to firm up.
Place the sesame seeds in a large bowl. Line two large baking trays with baking paper.
Roll 2 tbsp of the cookie dough into a ball, then roll in the sesame seeds to coat and place on one of the prepared trays. Using your hand, lightly flatten the cookie to a 2½ in (6 cm) circle. Repeat with the remaining dough to make 14 cookies, then place the cookies in the fridge for another 30 minutes.
Meanwhile, preheat the oven to 340°F (170°C).
For soft cookies, bake for 12 minutes or until lightly golden; for slightly crunchy cookies, bake for 14 minutes.
The cookies can be stored in an airtight container for 5 to 7 days. If you only want to bake half the cookies, store the remaining rolled uncooked cookies in the freezer for up to 3 months.
Notes
The cookie dough can be made ahead and stored in the freezer for a rainy day. Thaw the dough and follow the same cooking instructions.