Grease and line a 25cm x 30cm (10-inch x 12-inch) lamington tray or slice tin with baking paper.
Heat the oven to 180ºC / 350˚F. To make the base, combine the coconut, brown sugar, self-raising flour and melted butter. Press the mixture into the lined tray, using the back of a wooden spoon to smooth out the surface. Bake for 8-10 minutes or until the base is a nice golden brown. Allow to cool for about 10 minutes.
For the filling, combine the condensed milk, golden syrup and butter in a small, heavy-based saucepan and heat gently on medium heat. Stir constantly until the butter melts and the mixture comes to a simmer (you need to keep stirring, otherwise the mixture will burn on the bottom). Add your desired amount of salt. Pour onto the base, and return to the oven for another 10 minutes or until the caramel is golden and bubbly.
Top with melted dark chocolate, smoothing with a flat spatula. Once the chocolate has set, cut into squares.