Preheat oven and prep pan. Preheat the oven to 180°C (350°F). Grease a 23 x 23-cm (9 x 9-inch) baking pan and line with parchment paper. Allow the parchment paper to overhang the sides to make it easier to lift the shortbread out once cooked.Make shortbread dough. In the bowl of a stand mixer fitted with a paddle attachment, add the flour, butter, sugar, egg yolk, vanilla and salt and beat on medium speed until combined and a dough starts to form.
Level in pan and bake. Tip the dough into the prepared pan and using your hands, press the mixture into the pan until totally level. Using the base of a measuring cup or glass might help to level at the end. Prick the base with the tines of a fork. Bake for 20 to 25 minutes, or until golden. Remove the pan from the oven and allow it to cool completely.
Make caramel. In a medium saucepan over low heat, add the condensed milk, butter, sugar, golden syrup, espresso powder, and salt and heat until the sugar has dissolved, and the butter has melted. Increase the heat, and bring the mixture to a boil, stirring continuously so that it doesn’t catch. The caramel will thicken to a soft fudge consistency and turn golden-brown. This can take about 5 minutes. (If you have a candy thermometer then the caramel temperature should reach 107C (225F).
Pour over shortbread. Pour the caramel onto the cooled shortbread base, level with an offset spatula and leave to set for 1 hour in the fridge.
Melt dark chocolate and layer. Melt the dark chocolate and butter in a bowl over a pan of simmering water, ensuring the bottom of the bowl does not come into contact with the water. Once melted and glossy, pour the chocolate mixture over the caramel.
Melt white chocolate and decorate. Melt the white chocolate in the microwave in 20-second increments, stirring well after each. Spoon melted white chocolate into a small piping bag and pipe horizontal straight lines about ¼ inch (6 mm) apart over the dark chocolate. Score the lines using a skewer by dragging it up and down the top of the chocolate, perpendicular to the lines that you piped.Chill and cut. Chill in the fridge until completely set, at least 1 hour or at room temperature for a couple of hours. Remove from the pan by using the parchment paper. Cut into slices or squares using a sharp, hot knife, wiping between each cut.
Step-by-Step Photos: My blog post includes helpful step-by-step photos to help guide you through making this recipe.To Store: These shortbread bars can be stored at room temperature in an airtight container for up to 1 week.To Freeze: Millionaire's Shortbread Bars freeze really well. Wrap the individual bars in plastic wrap and foil and freeze for up to 3 months. Thaw completely at room temperature.Option:This makes an extra-thick layer of caramel. If you prefer a thinner layer, halve the caramel ingredients.Variation: For regular millionaire shortbread bars, omit the espresso from the caramel. TIP 1: Use a paper sling. Make life easier by hanging the paper over the edges of the pan. Gives you an instant sling! TIP 2: Use a heavy-based saucepan. Make your caramel in a heavy-based pan to minimise your chances of it catching. TIP 3: Stir the caramel continuously. Don't. Stop. Stirring. Leave your caramel for a minute, and by the time you've returned, it will have caught. So stir, stir, stir! TIP 4: Don't scrimp on the chill time. Refrigeration is key to allow the caramel to cool down and set. This means that when you slice it up, you won't have puddles of caramel oozing out the edges. TIP 5: Use a hot blade to cut the chocolate get photo-worthy clean cuts.