Millionaire's Shortbread

Ingredients

• Nonstick cooking spray
• 170g all-purpose flour (6 ounces; 1 1/3 cups)
• 75g rice flour or cornstarch (2 3/4 ounces; 2/3 cup)
• 56g golden caster, caster, or superfine sugar (2 ounces; about 1/4 cup plus 2 teaspoons)
• 1/4 teaspoon Diamond Crystal kosher salt; for table salt, use half as much by volume
• 150g unsalted butter (5 1/4 ounces; 11 tablespoons), melted
• 170g unsalted butter (6 ounces; 12 tablespoons)
• 450g dark brown sugar (about 15 3/4 ounces; about 2 cups plus 2 tablespoons)
• 1 1/2 cups (355ml) heavy cream
• 2 teaspoons Diamond Crystal kosher salt; for table salt, use half as much by volume
• 198g dark chocolate (7 ounces; about 1 1/4 cups), between 64 and 70% cacao, coarsely chopped
• 1 tablespoon (14g) unsalted butter
• Flaky sea salt, to garnish

Instructions

1. For the Shortbread: Adjust oven rack to middle position and preheat oven to 350ºF (175ºC). Line a 9- by 13-inch rimmed baking sheet with an 11- by 17-inch piece of parchment crosswise so the parchment overhangs the rim of the baking sheet by at least 2 inches. (If using pre-cut parchment, you may have to layer two pieces to create the overhang.) Using nonstick cooking spray, grease parchment and baking sheet; set aside.
2. In a large bowl, whisk together all-purpose flour, rice flour or cornstarch, sugar, and salt. Pour melted butter into flour mixture and, using a rubber spatula or dough scraper, incorporate butter into dry ingredients, gently stirring and pressing to combine, until a cohesive dough just forms and no dry bits remain, about 1 minute. (Be careful not to overwork the dough.)
3. Scrape dough into the prepared baking sheet and, using an offset spatula or the flat bottom of a glass or measuring cup, press dough into an even layer. Bake until fragrant and light golden, about 30 minutes. Remove shortbread from oven and set on wire rack to cool to room temperature, about 30 minutes.
4. For the Butterscotch: In a 6-quart pot, melt butter over medium heat. Whisk in brown sugar and salt cook, stirring occasionally, until sugar has melted and begins to bubble, about 5 minutes. Add heavy cream (it will bubble and sputter) and whisk until a loose sauce forms. Bring to a boil over high heat and cook, stirring occasionally, until sauce is thick, shiny, and registers 235ºF (113ºC) on an instant-read thermometer. Remove from heat and allow butterscotch to slightly cool, about 5 minutes. (It should no longer be scorching hot and still be warm enough to pour.)
5. Carefully pour butterscotch over cooled shortbread. Allow butterscotch to sit until warm room temperature, another 30 minutes, then refrigerate uncovered until just set, about 1 hour. Transfer baking sheet to freezer while you prepare the chocolate topping, at least 10 minutes and up to 1 hour.
6. For the Topping: In a medium heat-proof mixing bowl, combine dark chocolate and butter. Set it over a saucepan of simmering water, making sure the bottom of the bowl doesn't touch the water, and heat, stirring occasionally, until chocolate and butter have melted and mixture is homogenous, about 3 minutes. Remove bowl and set on a heatproof work surface. (Alternatively: In a medium microwave-safe bowl, heat dark chocolate and butter on medium-low power, stopping and using a heat-proof flexible spatula to stir every 30 seconds, until completely melted, about 3 minutes.)
7. Remove shortbread from freezer and pour chocolate over shortbread, using an offset spatula to evenly coat butterscotch. Refrigerate until set, about 30 minutes.
8. Lift parchment overhang to gently remove millionaire’s shortbread from rimmed baking sheet and transfer to a cutting board. Using a sharp knife, cut bars into squares, rectangles, or desired shape. Serve cold or at room temperature.