TAHINI BILLIONAIRE BARS

Steps:

• Shortbread:
• Place a rack in middle of oven; preheat to 350°F. Lightly coat a 9x9" or 8x8" baking pan, preferably metal, with nonstick spray. Line with parchment paper, leaving overhang on all sides. Whisk flour, powdered sugar, sesame seeds, and salt in a medium bowl. Add butter and toss to coat. Using your hands, work in butter until pieces are about pea-size. Using a rubber spatula, mix in yolks. (Dough should hold together when squeezed.)
• 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).
• Bake shortbread until golden brown, 24-28 minutes (or up to 35 minutes if using a glass dish). Let cool slightly.
• Butterscotch and assembly:
• While the shortbread is cooling, heat brown sugar and butter, whisking constantly, in a heavy medium saucepan over medium until butter is melted and sugar is dissolved, about 3 minutes. Whisk in cream and salt. Bring to a boil and cook, whisking constantly, until butterscotch is thick enough to coat a spoon (bubbling will become less frequent), about 5 minutes. Remove from heat, add tahini and vanilla and whisk until incorporated and smooth.
• Pour butterscotch over shortbread and tilt pan to distribute evenly. Chill until set, 30-40 minutes.
• Melt chocolate in a microwave-safe bowl in the microwave in 40-second increments, stirring between bursts. (Alternatively, place chocolate in a heatproof bowl set over a saucepan of gently simmering water; bowl should not touch water. Heat, stirring often, until melted and smooth.) Pour over butterscotch and spread evenly to edges of pan; sprinkle with sesame seeds. Chill until chocolate is firm, 1-2 hours.
• Using parchment paper overhang, pull bars out of pan and set on a cutting board; remove parchment. Using a serrated knife, slice into 25-36 pieces (5-6 cuts in each direction). Bring to room temperature before serving.
• Do Ahead: Bars can be made 4 days ahead. Store airtight at room temperature.