Step 1

Place a rack in middle of oven; preheat to 350°. Lightly coat a 9x9” or 8x8” baking pan, preferably metal, with nonstick cooking oil spray or vegetable oil. Line with parchment paper, leaving overhang on all sides. (If you have a precut sheet of parchment, simply slice it in half crosswise, then overlap the two halves to line the pan.)

Step 2

Whisk 1 cup (125 g) all-purpose flour, ¾ cup (83 g) powdered sugar, ⅓ cup toasted sesame seeds, and ¾ tsp. Diamond Crystal or ½ tsp. Morton kosher salt in a medium bowl. Cut ½ cup (1 stick) chilled unsalted butter into ½” pieces and add to dry ingredients; toss to coat. Using your fingers, work in butter until pieces are about pea-size.

Step 3

Separate 2 large eggs—the easiest way to do this is to crack open eggs one at a time and gently pour into a (clean) hand, letting whites fall through your fingers either into the sink or a small bowl while trapping the yolk. Add yolks to bowl with flour mixture and mix in with a rubber spatula or wooden spoon. Dough should hold together when squeezed in your hand.

Step 4

Scrape dough into prepared pan and gently press into an even layer (lightly flour your hands if needed). Prick dough with a fork in several spots (this will help it stay flat as it bakes). Lightly flour the fork if it's sticking to the dough.

Step 5

Bake shortbread until golden brown, 24–28 minutes (or up to 35 minutes if using a glass dish). Let cool slightly.

Step 6

While the shortbread is cooling, make the tahini butterscotch. Cut remaining ½ cup (1 stick) chilled unsalted butter into 8 pieces. Heat butter and 1¼ cups (packed; 250 g) light brown sugar, whisking constantly, in a medium heavy saucepan over medium until butter is melted and sugar is dissolved, about 3 minutes. Whisk in ¾ cup heavy cream and 2 tsp. Diamond Crystal or 1 tsp. Morton kosher salt. Bring to a boil and cook, whisking constantly, until bubbling becomes less frequent and butterscotch is thick enough to coat a spoon, about 5 minutes. (When in doubt, err on the side of giving the mixture another 30 seconds—it's better if it's a little too firm than too runny.)

Step 7

Remove from heat, add ½ cup tahini and 1 tsp. vanilla extract and whisk until incorporated and smooth. Pour butterscotch over shortbread and tilt pan to distribute evenly if necessary. Chill until set, 30–40 minutes.

Step 8

Coarsely chop 6 oz. bittersweet chocolate (65%–75% cacao) and place in a microwave-safe bowl. Melt in a microwave in 40-second increments, stirring between bursts. (Alternatively, place chocolate in a heatproof bowl set over a saucepan of gently simmering water a.k.a. a double boiler; do not let bowl touch water. Heat, stirring often, until melted and smooth.) Pour over butterscotch and spread evenly to edges of pan with a small offset spatula or rubber spatula; sprinkle with toasted sesame seeds. Chill until chocolate is firm, 1–2 hours.

Step 9

Using parchment paper overhang, pull bars out of pan and set on a cutting board; remove parchment. Using a serrated knife (a chef's knife will smoosh these), slice into 25–36 pieces (5–6 cuts in each direction) with a sawing motion. If you're having trouble slicing, feel free to freeze the bars for 20–30 minutes, which will harden the caramel and ease the process.

Step 10

Do ahead: Bars can be made 4 days ahead. Store airtight at in the fridge.