Preheat your oven to 160°C (320°F). Line a square baking pan with parchment paper, leaving overhang for easy removal.
In a mixing bowl, combine the flour, caster sugar, and a pinch of salt. Use your fingertips to rub in the cold butter until the mixture resembles coarse crumbs—this gives the shortbread its tender, sandy texture.
Press this crumbly mixture evenly into the prepared pan, smoothing the surface with the back of a spoon. Bake for 20-25 minutes, until lightly golden around the edges and fragrant.
While the shortbread bakes, make the caramel: In a heavy-bottom saucepan, stir together golden syrup, sweetened condensed milk, and butter. Place over medium-low heat, stirring constantly, until the mixture thickens, deepens in color, and turns a rich amber—about 10-15 minutes.
Remove the baked shortbread from the oven and let it cool slightly in the pan. Once warm, pour the hot caramel evenly over the shortbread, spreading gently with a spatula to cover the surface completely.
Allow the caramel to cool at room temperature for about 30 minutes, then transfer the pan to the fridge. Chill for at least 2 hours until the caramel is firm and set.
Using the parchment overhang, lift the set caramel shortbread from the pan. Place on a cutting board and cut into squares or rectangles with a sharp knife.
If desired, sprinkle a pinch of sea salt on top of the caramel for a flavor boost. Serve at room temperature for the perfect balance of chewy caramel and crumbly shortbread.
