How to Make It
Prepare the Sesame Shortbread Layer: Preheat oven to 325°F. Lightly grease and line a 9-inch square baking pan with parchment paper, allowing 1 to 2 inches of paper to hang over edges of pan to use as “handles” later.
Beat together butter and sugar in a large bowl with an electric mixer at medium speed until pale and fluffy, 2 to 3 minutes. Add egg yolk and vanilla; beat until combined. Add flour, sesame seeds, and salt to bowl; beat on low speed until combined and sandy.
Add shortbread mixture to prepared pan, and using moistened hands, press into an even layer. Bake in preheated oven until light golden brown, 30 to 35 minutes. Transfer pan to a wire rack to cool.
Prepare the Tahini Caramel Layer: Combine condensed milk, butter, corn syrup, honey, and salt in a medium saucepan over medium. Cook, stirring constantly, scraping bottom and sides of pan well, until mixture thickens, is dark golden in color, and reaches 220°F on a candy thermometer, 15 to 20 minutes. Turn off heat; stir in tahini and vanilla. Immediately pour caramel evenly over cooled crust, and spread in an even layer using a heatproof spatula. Allow caramel layer to cool and set slightly, about 15 minutes.
Prepare the Chocolate Layer: Melt chocolate over a double boiler, stirring constantly, until melted and smooth, about 5 minutes. Pour melted chocolate over caramel, and spread into an even layer. Sprinkle evenly with toasted sesame seeds and flaky salt. Chill until chocolate is set, 30 to 45 minutes. Use parchment paper overhangs to lift shortbread from pan. Transfer to a cutting board, and cut into 2-inch bars using a chef’s knife. Store leftovers in an airtight container.