Preheat the oven to 325ºF. Line 2 to 3 rimmed baking sheets with parchment paper; set aside.

Add the butter and tahini to a deep bowl. Beat, using an electric mixer, until creamy, 30 to 60 seconds.

Add the sugar, and beat on medium speed until creamy, 3 to 5 minutes.

Add the egg, egg yolk, and vanilla. Beat until thickened and creamy, 2 to 3 minutes.

Combine the flour, baking soda, baking powder, and sea salt in a small bowl. Whisk to blend. Add to the butter-tahini mixture. With the mixer on the lowest speed, mix in the flour until just blended in. The dough will be very soft.

Use a rubber spatula to fold in the chocolate chips.

Scoop the dough, 2 tablespoonfuls at a time (a cookie scoop is handy here!) onto the prepared sheet pans, 2-inches apart. Bake one tray at a time, until the cookies are barely golden around the edges, and still puffed in the center, 10 to 12 minutes.

Remove cookies from the oven, and sprinkle immediately with flaky sea salt. Let them cool on the tray for 2 minutes. Transfer to a wire rack to cool completely.

Notes: For a crisper cookie, bake 15 minutes. Dough made me made up to 3 days in advance, and baked as needed.