1. In the bowl of a stand mixer, beat the butter, tahini, granulated sugar and brown sugar on medium speed for 2 to 3 minutes, until fluffy. (The dough can also be made in a large mixing bowl, stirred with a spatula.)
2. Stop the mixer and scrape down the sides. Add the egg, the yolk, and vanilla, and continue to mix for another minute, stopping the mixer to scrape down the sides of the bowl during mixing, to make sure the eggs are getting incorporated.
3. In a small bowl, whisk together the flour, baking soda, and kosher or sea salt. With the mixer on low speed, add the dry ingredients until just combined, then add the chocolate chips. Do not overmix. Cover the dough and refrigerate overnight.
4. Preheat the oven to 325ºF (160ºC). Line two baking sheets with parchment paper or silicone baking mats.
5. Form the cookies into rounds using an ice cream scoop, or your hands. For small cookies make each 1 1/2-inch (3,5cm), for larger cookies, make them 2-inches (5cm) round. Place them evenly spaced on the baking sheets, 3-inches (8cm) apart). Bake one sheet at a time, so you can keep an eye on them, in the middle rack of the oven.
6. Bake the cookies, turning the baking sheet in the oven midway during baking, until the cookies are golden brown around the edges but still pale in the center. For small cookies, about 12 minutes, for larger cookies, about 14 to 15 minutes. Remove from the oven, sprinkle cookies with a bit of flaky sea salt, and let the cookies cool on the baking sheet. Bake the remaining cookies the same way.