In a large mixing bowl or the bowl of a stand mixer, whip the butter on high speed until light and fluffy.

Add the dark brown sugar, granulated sugar, and tahini, and mix until well combined.

Mix in the egg and vanilla bean paste.

In a small bowl, whisk together the flour, baking soda, and kosher salt

Add to the large bowl and mix on medium speed until just barely combined.

With a wooden spoon or a rubber spatula, fold in the chopped dark chocolate.

Cover bowl with clingfilm/plastic wrap and transfer to the refrigerator to chill, at least 8 hours, or overnight.

Preheat oven to 176° C / 350° F. Line a half-sheet rimmed tray with parchment.

With a 1 1/2-inch cookie scoop, scoop the dough into portions. Press reserved chocolate chunks on tops of each portion to decorate.

Place on prepared tray, allowing room for the cookies to spread as they bake.

Press a few additional chunks of chocolate into the tops of each cookie.

Bake in preheated oven for 12-13 minutes, or until lightly golden brown. For perfectly round cookies, immediately reshape with a round metal cutter.

Cool on the cookie sheet for 10 minutes, then transfer to a wire rack to cool completely. Repeat with remaining cookie dough.

Layer tahini cookies between sheets of parchment and store in an airtight container at room temperature for up to three days. Tahini chocolate chunk cookies can also be frozen.