Scottish shortbread is a buttery, crumbly cookie that melts in your mouth. Topped with thick caramel and chocolate and you have Millionaire Shortbread Bars. Partially adapted from America's Test Kitchen.
Pre-heat oven to 325°F. Line a 13x9" baking pan with parchment paper or aluminum foil, leaving some hanging over edges of pan to lift out after baking.
In a bowl or mixer bowl combine flour, powdered sugar, and kosher salt. Add melted butter and stir with rubber spatula or mix until flour is mixed and dough is crumbly.
Press evenly over bottom of prepared pan. Using fork, pierce dough at 1-inch intervals all over. Bake for 30 minutes, or until light golden brown and firm to touch.
Remove from oven. While still warm, use a flat metal spatula and press surface of shortbread slightly to compress (optional). I find this helps when cutting later. Set aside.
Stir all caramel ingredients together in large, heavy-bottomed saucepan. Cook over medium to low heat, stirring frequently with heat-proof rubber spatula to avoid any scorching on bottom for 20 minutes, or until mixture reaches 240°F (soft ball stage on candy thermometer).
Carefully pour over shortbread while hot and spread to even thickness. Let cool completely for 1 1/2 hours, or until caramel is firm to the touch.
In a small microwave safe bowl melt the chocolate and oil in microwave for 30 second intervals on high, stirring in between until melted. Stir to incorporate the oil until smooth.
Pour over caramel and shake pan to cover to edges and look smooth or use a spatula or knife and spread back and forth across the surface (as pictured). Refrigerate shortbread until chocolate is set, about 10-15 minutes.
Lift out of pan using parchment overhang onto a cutting board. Slice lengthwise down the middle and cut each half into 8 bars each for a total of 16 rectangle bars, or cut into squares.
Golden Syrup is a British syrup made from cane sugar. The brand I know of called Lyle’s Golden Syrup is found near the honey and maple syrup in some supermarkets, but if you can't find it, you can order it online or substitute corn syrup.