Preheat your oven to 160°C (320°F) and line a rectangular baking tin (approximately 20x30 cm) with parchment paper, leaving some overhang for easy removal later.

In a mixing bowl, beat together the softened unsalted butter and caster sugar until the mixture is light and fluffy.

Sift in the plain flour and mix until just combined. The dough will be crumbly but should hold together when pressed.

Transfer the mixture into the prepared baking tin and press it down evenly to form a solid base. You can use the back of a spoon or your hands for this.

Bake in the preheated oven for about 20-25 minutes or until the edges are lightly golden. Remove from the oven and let it cool completely in the tin.

In a saucepan over medium heat, combine the unsalted butter, condensed milk, golden syrup, and salt. Stir continuously to mix well.

Bring the mixture to a gentle boil, stirring constantly. Continue to cook for about 5-10 minutes until it thickens and turns a golden caramel color. Be cautious not to burn it; you want it thickened but smooth.

Once thickened, remove the caramel from the heat and pour it over the cooled shortbread base, spreading it evenly. Allow it to cool and set at room temperature or refrigerate it for quicker setting.

In a microwave-safe bowl or over a double boiler, melt the milk chocolate, stirring until it's smooth.

Pour the melted milk chocolate over the set caramel layer, spreading it evenly to cover the surface entirely.

If using, melt the white chocolate and drizzle it over the milk chocolate layer for an attractive finish. Swirl the white chocolate into the milk chocolate using a toothpick or the end of a cake skewer.

Refrigerate the assembled shortbread until the chocolate is set, typically for at least 2 hours.

Once everything is set, use a sharp knife to cut the layered dessert into squares or rectangles. Use the parchment overhang to lift it out of the tin for easier cutting.

Store the caramel shortbread in an airtight container in the refrigerator. They can last up to a week, but they are best enjoyed within a few days for optimal texture.