In a large bowl or your stand mixer (which is what I use), mix the water and flax seed together to prepare the flax egg and set aside for five minutes. 

Add in the tahini, maple syrup, and vanilla and whisk together. Add the flour, cocoa powder, sugar, baking powder, baking soda, cinnamon, and salt.

Stir until fully combined. Fold in chocolate chips. The dough will likely be fairly wet and very sticky at this stage.

Cover the bowl with plastic wrap and chill in refrigerator for at least 1 hour.

Preheat your oven to 350 F and line a cookie sheet with parchment paper.  Place the sesame seeds and sea salt in a small bowl and sir to combine. Fill another small bowl with water.

The cookie dough will still be sticky, so wetting your hands periodically will help. Using a small spoon (like the regular one you eat with), scoop cookie dough and gently roll into a loose ball with your hands. Dip the tops (half of the ball) into the sesame mixture and place on cookie sheet.

Dip a fork into the water and press down gently in a criss-cross pattern. Rewet your fork each time. This will help the cookies spread more evenly as they bake (not to get a special pattern). 

Bake for 8 to 11 minutes (9 is usually about perfect for me).  Remove from oven and bang gently on the counter, then let cool for a few minutes before transferring to a wire rack to finish cooling. 

These will keep well for a few days...I've found that they actually do fairly well when exposed to a bit of air and actually get stale in an airtight container.  I've left them out on the counter overnight.  So I'd keep them in a container but with the lid ajar, and consider layers of parchment between the cookies.  

As with many gluten-free bakes, I've found that I love them same-day but not as much fresh out of the oven.  They benefit from setting up completely and cooling down, the texture is better and I feel like I can really get the subtle flavors better as well.