Line your baking tin with parchment paper, leaving overhang for easy removal, and preheat your oven to 160°C (320°F).
In a mixing bowl, cream together softened butter and golden caster sugar until the mixture is light and fluffy, about 2 minutes. Add vanilla extract and mix well.
Sift in the flour and a pinch of salt, then gently fold everything together until just combined. Press this dough evenly into the prepared tin, smoothing the surface with the back of a spoon or your fingers.
Bake the shortbread base for 20-25 minutes until it turns golden around the edges and feels firm when lightly pressed. Remove from the oven and let it cool slightly in the tin for about 10 minutes.
Meanwhile, prepare the caramel: in a saucepan over low heat, combine sweetened condensed milk, dark brown sugar, and butter. Stir constantly until the butter melts and the mixture starts to bubble gently, about 3-4 minutes.
Increase the heat to medium and cook, stirring continuously, until the caramel thickens, turns a deep amber color, and smells sweet and caramel-y, about 8-10 minutes. Once ready, pour the hot caramel over the slightly cooled shortbread base, spreading it evenly with a spatula.
Bake the assembled treat for an additional 15 minutes until the caramel is bubbling at the edges and just starting to set. Remove from oven and allow to cool completely in the tin.
Once cooled, refrigerate for at least 2 hours or until the caramel is firm. Use the parchment overhang to lift the block out of the tin, then cut into squares with a sharp knife, wiping the blade between slices for clean cuts.
Sprinkle with a few flakes of sea salt for contrast, if desired, and serve. Store leftovers in an airtight container in the fridge for up to 5 days.
