In the large bowl of a stand mixer with paddle attachment, beat brown sugar and butter together until light and fluffy, scraping down the sides as needed, 2 mins.
Add tahini and vanilla and beat 2 mins more.
Add both flours and mix on low until there are no more dry spots of flour, again scraping down the sides as you go. If the dough seems too soft and/or sticky add more all purpose flour as needed (max 1/4 cup).
Fold in chocolate
Turn the dough out onto a a large peice of parchment paper or plastic wrap. Using your hands, press dough together, so there are no air pockets and roll into a log that's about 12 inches long and 2 inches in diameter.
Wrap the dough log tightly with parchment (or plastic wrap) and refrigerate from 4 hours to 4 days. (I twist the ends of the parchment or plastic like a candy wrapper to ensure a tight fit.)
20-30 minutes before you want to make your cookies. Pop the dough log into the freezer. (I found this helpful because the dough is quite soft and it's hard to cut into even disks, unless it's more solid)
Preheat oven to 350F and line a large baking sheet with parchment paper.
Unwrap chilled dough and brush with the beaten egg.
Place the sesame turbinado sugar in a large dish (long enough for your dough). Combine with your hands.
Roll your dough log onto the mixture, pressing with your hands to make sure the dough is evenly coated.
Using a sharp knife, cut cookies into 1/2" rounds. The trick here is to use a very sharp knife, saw back and forth quickly while pressing down. The sawing motion will help cut through the chocolate chips without causing the whole cookie to crumble. Alternately, on Half Baked Harvest they say to cut in a quick forward motion but this did not work for me.If pieces crumble off, simply smoosh it back together to reform the cookie. It'll bake-up just fine! Re-roll the edges in the sugar/sesame mixutre if some of it has fallen off. Place cookies about 1 inch apart on the cookie sheet and bake for 10-12 minutes until golden
Remove cookies from the oven and immediately sprinkle with flakey salt. (I concentrate on sprinkling the salt over the chocolate chips)
Let cookies cool for at least 10 minutes as they are quite delicate. Before storing them, make sure they cool completely at room temperature. It takes a while for the chocolate to set.They can be stored in an airtight container for 4 days.