NOTE: I used two Theo 70% Pure Dark chocolate bars, roughly chopped
Whisk the flour, baking soda and salt together in a medium bowl. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, brown sugar and granulated sugar at medium speed just until smooth, 30 to 60 seconds. Do not beat until pale and fluffy as this will cause your cookies to incorporate more air and spread in the oven.
Add the egg, yolk and vanilla and beat until the egg is incorporated, 30 to 60 seconds more. Add all of the flour mixture and mix on low speed until halfway incorporated. Add the sesame seeds, chocolate and pistachios and mix on low speed until the dough just comes together. Take care not to overwork the dough or your cookies will be tough.
Use a 1/4-cup scoop to form balls of dough. Place the formed dough on a parchment-lined sheet pan. Refrigerate overnight. I know this seems fussy and unfair considering how good this cookie dough is, but resting your dough before baking it off is the single most effective way to take your cookies to next-level, eye-rolling deliciousness. Trust me; it's worth it.
You can also freeze the dough balls. Once they are frozen solid, remove them from the sheet pan and store them in a resealable plastic bag in the freezer for up to two months. I always freeze my dough and bake from frozen as it makes for prettier cookies that hold their shape better in the oven.
When you are ready to bake off the cookies, preheat the oven to 325°F. Place racks in the center and upper third of the oven and line two sheet pans with parchment paper.
Place 6 dough balls on each of the prepared sheet pans, leaving at least 2 inches of space around each cookie. (You can freeze the last 4 dough balls for emergency cookie cravings or bake them as a second batch.) Sprinkle the dough balls with coarse salt, if desired.
Bake for about 18 minutes, until lightly browned around the edges and still soft in the middle (add 3 to 4 minutes if baking from frozen). If you're unsure when to take them out, err on the side of under- rather than overbaking them. Remove the cookies from the oven and allow them to cool on the baking sheet for about 3 minutes, then transfer them to a wire rack to cool completely. These cookies will keep for up to 4 days, stored in an airtight container.