Preheat your oven to 180°C (160°C fan). Line your baking tin with parchment paper, leaving an overhang on the sides.
In a large bowl, combine the flour, caster sugar, and pinch of fine salt. Add the cold cubed butter and rub it into the dry ingredients using your fingers or a pastry cutter until the mixture resembles coarse breadcrumbs.
Press the crumb mixture firmly into the base of the prepared tin. Bake for 20 minutes or until the shortbread is lightly golden. Remove from the oven and set aside.
While the base cools, combine the condensed milk, brown sugar, 100g of butter, and golden syrup in a heavy-bottomed saucepan over medium-low heat.
Stir constantly until the butter has melted and the sugar has dissolved. Bring the mixture to a gentle boil, then reduce the heat to low.
Continue to cook the caramel, stirring continuously, for about 8 to 10 minutes. The mixture should thicken significantly and turn a deep golden color. Be careful not to let it burn on the bottom of the pan.
Pour the caramel over the baked shortbread base and spread it evenly using an offset spatula.
Sprinkle the flaky sea salt over the top of the warm caramel.
Let the tray cool completely at room temperature, then refrigerate for at least two hours to allow the caramel to firm up before slicing into 16 squares.