Step 1

Beat sugar, tahini, butter, and egg yolk with a stand mixer fitted with a paddle attachment on medium speed until smooth and pale, 4 to 5 minutes, stopping to scrape down sides of bowl as needed.

Step 2

Stir together flour and kosher salt in a medium bowl. With mixer running on low speed, gradually add flour mixture to egg mixture, beating until combined, about 1 minute (mixture will be crumbly but should hold together when squeezed). Transfer half of the dough (about 1 1/2 cups) to a medium bowl; set aside. Add cocoa to remaining dough in bowl, and beat on low speed until combined, about 1 minute.

Step 3

Preheat oven to 325°F. Line bottom of a straight-sided 13- x 9-inch metal pan with parchment paper. Spoon alternating clumps of varying sizes of each dough into bottom of prepared pan to create a marbled pattern. Using a flat-bottomed glass or measuring cup, press dough evenly onto bottom of pan (it should be about 1/4 inch thick). Using a sharp knife, cut the dough all the way through into 18 (about 4 1/2- x 1 1/4-inch) bars. Freeze until firm, about 15 minutes. Sprinkle top with flaky sea salt.

Step 4

Bake in preheated oven until top is light golden and begins to look dry, 30 to 40 minutes, rotating pan 180 degrees on rack every 10 to 15 minutes. Transfer pan to a wire rack. Immediately re-cut the cookies along the lines. Let cool in pan completely, about 1 hour. Store in airtight container at room temperature up to 4 days.