A traditional buttery Scottish shortbread covered by a thick layer of caramel and finished off with a chocolate glaze. Three layers of deliciousness! These bars are R.I.C.H, hence the name Millionaire. 😉
To make the shortbread base, preheat the oven to 350℉. Butter the bottom and sides of a 9×13 inch metal baking pan. Put a long piece of parchment paper in the bottom of the pan, letting the parchment extend up two sides of the pan and overhang slightly on both ends. Place another piece of parchment paper in the same manner, perpendicular to the first sheet. (This will make it easy to remove the bars from the pan after they have baked.) Butter the parchment.
In the bowl of an electric mixer fitted with the paddle attachment, cream the sugar and butter together, about 2 minutes.
Add 2 cups of the flour and beat until well combined. Add the egg yolk and beat until just combined, about 30 seconds.
Turn the dough out onto a lightly floured work surface. Dust the top of the dough and your hands with a little flour. Use your hands to gently work the dough into a 6-by-6-inch square. You will have to turn the dough and sprinkle the top with flour as you go. Sprinkle the remaining 1/2 cup flour on the surface of the dough. Fold the dough over and knead until incorporated, then flatten the dough into rectangle. Transfer the rectangle to the prepared pan and press it into the pan.
Prick the dough all over with a fork and bake in the center of the oven for 20-22 minutes, until golden brown. Transfer to a wire rack to cool completely.
To make the caramel filling, place the sweetened condensed milk in an oven-proof dish and sprinkle with some kosher salt. Tightly cover with foil.
Place the covered dish in a larger roasting or casserole pan and fill it up with water until it reaches three-quarters of the way up the covered dish to create a water bath. Bake at 425 degrees F for 60-90 minutes stirring every 30 minutes and checking on the water level, adding more as needed, until it is brown and caramel-like. Remove from the oven and whisk until smooth.
Pour the caramel filling over the cooled shortbread and place the pan in the refrigerator until cool, about two hours.
To make the chocolate glaze, in a large bowl, combine the chocolate, corn syrup, and butter. Set the bowl over a saucepan of simmering water and stir with a rubber spatula until the mixture is completely smooth. Remove the bowl from the pan and stir for 30 seconds to cool slightly. Pour the mixture over the chilled caramel layer and use an offset spatula to spread it into an even layer.
Put in the refrigerator for one hour, or until the glaze hardens.
Remove the pan from the refrigerator 30 minutes before serving so as not to crack the chocolate glaze. Cut into squares and serve.