Whisk together flour and salt in a bowl.
Beat butter and tahini with an electric mixer on medium speed until creamy, 1 to 2 minutes.
Add sugar and beat until light and fluffy, 1 to 2 minutes.
Beat in egg until combined.
Reduce mixer speed to low and gradually beat in flour mixture just until combined.
Remove two-thirds of dough.
Beat cocoa powder into remaining dough just until combined.
Line a rimmed baking sheet with parchment paper.
On a lightly floured work surface, roll each dough portion into a 10 ½-inch circle (about ¼ inch thick).
Stack chocolate dough on top of plain dough.
Cut circle in half and stack one half onto the other.
Using your hands, gently fold and knead stack into a marbled ball.
Shape into a 12-by-8-inch rectangle.
Place on prepared baking sheet; chill 1 hour.
Preheat oven to 350°F.
Line two baking sheets with parchment paper.
Cut dough into 1 ½-by- 2 ½-inch rectangles and roll edges in sesame seeds; place on prepared baking sheets.
Bake, in batches, until edges are lightly browned, 12 to 14 minutes.
Transfer to wire racks to cool.