Millionaire’s Shortbread

Preheat the oven to 350°F and set an oven rack in the middle position. Line a 9-inch square pan with aluminum foil, pushing the foil neatly into the corners and up the sides of the pan. Spray the foil with nonstick cooking spray or grease with butter.

In the bowl of a food processor fitted with a metal blade, combine the flour, brown sugar, cornstarch, and salt; process until well combined and no lumps of sugar remain. Add the butter and egg yolk; process for 10 to 15 seconds, until a coarse meal forms. Add the ice water and blend until you have a moist, crumbly dough, about 10 seconds more. Dump the dough into the prepared pan and press with your fingers into an even layer. Prick the dough all over with a fork and bake until golden, about 20 minutes. Set aside to cool.

In a medium, heavy-bottomed saucepan over medium heat, melt the butter. Add the sweetened condensed milk, heavy cream, corn syrup, vanilla, brown sugar, and salt, whisking until evenly combined. Bring to a simmer and continue cooking, stirring frequently, until the mixture registers between 236°F and 239°F on an instant-read digital thermometer, 10 to 20 minutes. Once the caramel reaches the desired temperature, carefully pour it over the crust. Tilt the pan to spread the caramel evenly. Let sit for 30 to 35 minutes, until the caramel layer is set but still slightly warm.

Place the semi-sweet and milk chocolate in a small microwave-safe bowl. Microwave in 20-second bursts, stirring in between, until the chocolate is about 75% melted. Continue melting in 5 to 10 second-bursts, stirring in between until the chocolate is melted and smooth. Using an offset spatula, spread the chocolate evenly over the caramel layer. Refrigerate until the bars are cold and the chocolate is set, 1½ to 2 hours.

Let the bars sit out for 10 to 15 minutes to soften the chocolate slightly. Lift the bars out of the pan using the foil overhang and transfer to a cutting board. Using a sharp knife, cut into small squares. Sprinkle the bars with flaky sea salt.