Place a rack in the upper third of the oven and preheat to 325 F. Line a baking sheet with parchment paper and set aside.

In the bowl of a stand mixer with paddle attachment, beat together the butter, tahini, and sugar.  Beat until well incorporated and slightly fluffy in texture, about 3 minutes.

Stop the mixer, scrape down the bowl and add the eggs and egg yolk.  Beat on medium speed until well blended, about 2 minutes.

Add the dry ingredients, all at once to the wet mixture and beat on low speed until JUST incorporated. Stop the mixer and remove the bowl from the mixer. 

Add the chopped pistachios and 1 tablespoon of sesame seeds and finish incorporating by hand with a spatula.  The dough may be slightly dry and that's okay, but you should be able to squeeze it and come together.

Divide the batter into two on the prepared baking sheet and shape into two logs, about 8 inches long and 1 1/2-inches wide. Brush with egg wash and sprinkle generously with the remaining sesame seeds.

Bake for 15 minutes, then rotate the pan and bake for another 15 minutes.  You want it golden brown and cooked through, but JUST barely. Remove from the oven and allow to cool on the baking pans until cool enough to handle, but still soft. Use a serrated knife to slice the biscotti into 1-inch thick pieces.

Place cut side down on the baking sheet and bake for another 15 to 20 minutes, flipping once during baking.  You want them to get good and crisp, so decide if they need more time in the oven. When done, remove from the oven and allow to cool. The biscotti will still be slightly soft.

Gently heat the dark chocolate chips with the coconut oil and stir until melted and creamy.  Dip the biscotti in it or drizzle with it, and then sprinkle with sea salt crystals.