Beat together the butter, tahini, and both sugars, scraping down side of bowl.
Add the egg, yolk and vanilla and beat until incorporated.
Beat in the baking soda and salt, then stir in the flour until incorporated. If you prefer to mix the dry ingredients together beforehand, that's fine too, but I had great results just mixing everything in one bowl.
Stir in the chocolate.
Portion the cookies into balls with a medium size cookie scoop and chill the portioned balls for about an hour or until ready to bake.
Preheat the oven to 325ºF (180ºC). Line two baking sheets with parchment appers.
Arrange the cookies on the baking sheets and bake one sheet at a time for about 14 minutes.
Alternatively, you can bake the cookies at 375 for 12 minutes for slightly crunchier cookies. Ovens may vary, but I kind of liked the 375 version the best.
Remove from the oven, sprinkle cookies with a bit of flaky sea salt, and let the cookies cool on the baking sheet.