Step 1

Preheat the oven to 350 degrees. Line a 9-inch square pan with aluminum foil, pushing the foil neatly into the corners and up the sides of the pan, using two pieces if necessary to ensure that the foil overlaps all edges (the overhang will help removal from the pan). Spray the foiled pan with nonstick cooking spray or grease with butter.

Step 2

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 brown sugar remain. Add the butter and pulse until a coarse meal forms. Add the ice water and egg yolk and blend until moist clumps form. Dump the dough into the prepared pan and press with your fingers into an even layer (dust your fingers with flour if the dough is too sticky). Pierce the dough all over with a fork and bake until golden, about 20 minutes. Set aside to cool.

Step 3

Whisk the sweetened condensed milk, brown sugar, butter, golden syrup, vanilla and salt together in a medium saucepan over medium heat until the sugar dissolves, the butter melts and the mixture comes to a boil. Attach a candy thermometer to the side of the pan and boil gently, whisking constantly, until the caramel is thick and the temperature registers 225 degrees F, about 6 minutes. Pour the caramel over the warm crust; cool for about 15 minutes, or until caramel is set.

Step 4

Place the chocolate and cream in a microwave-safe bowl. Microwave in 20 second intervals, stirring in between, until the chocolate is about 75% melted. Stir, allowing the residual heat in the bowl to melt the remaining chocolate, until smooth (if necessary, place the chocolate back in the microwave for a few more; just be sure not to overheat or the mixture will curdle). Spread the chocolate over the caramel layer. Refrigerate the bars until the chocolate is set, at least 1 hour. Using the foil overhang, lift the bars out of the pan and transfer to a cutting board. Cut into small squares and store in the refrigerator until ready to serve.

Step 5

Note: The shortbread layer will crumble a bit when you cut it; that's just the nature of it.

Step 6

Freezer-Friendly Instructions: These can be frozen for up to 3 months. After they are completely cooled, double-wrap them securely with aluminum foil or plastic freezer wrap. Thaw overnight on the countertop before serving.