Melt the chocolate in a microwave (in 20-second bursts, stirring after each one) or in a bowl set over a pot of barely simmering water. Scrape the melted chocolate into a large bowl.
Stir in the tahini, brown sugar, and cinnamon.
The add the milk and egg and stir until smooth.
Add the flour, baking powder, baking soda, and salt and stir until combined to a soft dough.
Cover the dough and chill it for at least one hour or up to 3 days.
If chilled for longer than one hour, remove the dough from the fridge about a half hour before you plan to use it so it softens enough to scoop.
Preheat the oven to 325℉ (170℃). Grease 1 or 2 cookie sheets or line them with parchment paper.
Roll about 2 tablespoons of cookie dough into balls or use a cookie scoop (size 30). Place the balls on the prepared cookie sheets and flatten them slightly so they are about as thick as a finger. Sprinkle each cookie with a pinch of sesame seeds, if desired.
Bake the cookies for 12 minutes, until they are puffed up in the centers, but still very soft when lightly touched (they will still feel like they're raw in the centers). They will firm up as they cool.
Allow the cookies to cool in the pans for 5 minutes, then move them to a cooling rack to finish cooling.
Notes
*Stir the tahini well before measuring it, to incorporate any oil separation.These cookies can also be frozen. Shape the dough into balls, flatten them slightly, and freeze them on a cookie sheet until firm. Then transfer the frozen cookies to a freezer container or ziptop freezer bag and freeze for up to 3 months. Bake the cookies from frozen in a 325℉ (170℃) oven for 15 minutes. This way you can bake only as many cookies as needed and always have fresh cookies within a few minutes.