Tahini, a rich paste made from ground sesame seeds, gives these chewy cookies a distinctive nutty flavor. It also makes the dough extra light and fluffy when you first put it together, so make sure to leave ample time to chill it before baking the cookies.
Whisk together the flour, baking powder, baking soda and salt in a medium bowl. Set aside.
Beat the brown sugar, butter, tahini and granulated sugar in a large bowl with an electric mixer on medium-high speed until pale and fluffy, about 5 minutes (use the paddle attachment if using a stand mixer). Beat in the eggs one at a time, then beat in the vanilla. Reduce the mixer speed to low. Add the flour mixture and beat until just combined. Stir in the chocolate chunks by hand. Press plastic wrap directly onto the surface of the dough and refrigerate until very firm, at least 6 hours and up to overnight.
Position oven racks in the upper and lower third of the oven and preheat to 375 degrees F. Line 2 baking sheets with parchment.
Stir together the white and black sesame seeds in a small bowl. Scoop a leveled 1/4-cup mound of the cookie dough (an ice cream scoop works well here), then dip the top of the mound into the sesame seeds and transfer to one of the parchment-lined baking sheets. Continue scooping until you have 6 mounds on each baking sheet. Refrigerate the remaining cookie dough.
Bake, rotating the baking sheets from front to back and top to bottom halfway through, until the cookies are golden around the edges but still soft in the middle, about 18 minutes. Let cool for 5 minutes on the baking sheets, then transfer to racks to cool completely. Allow the baking sheets to cool, then line with fresh parchment and repeat the process with the remaining dough.
When measuring flour, we spoon it into a dry measuring cup and level off excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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