Preheat oven to 170°C (fan forced) and line a 20 x 20 cm square cake tin with baking paper, leaving some overhang for easy removal.
Make the base. Melt the butter and golden syrup together until smooth. In a bowl, mix the flour, cocoa, coconut, brown sugar, and salt. Add the melted butter mixture and stir until it looks like damp sand. Press firmly into the base of the tin and smooth the surface (I use the back of a spoon for this).
Bake the base for 12 to 15 minutes until the edges are set but the centre is still soft. Remove and let cool slightly.
Make the caramel. Whilst the slice is cooling, place the condensed milk, golden syrup, and butter into a saucepan. Whisk over a medium-low heat for 7 to 10 minutes until the mixture has thickened slightly, and is glossy and light golden in colour. Remove from heat.
Pour the caramel over the warm base and spread it evenly. Return to the oven for 15 to 17 minutes to or until the caramel is rich golden brown in colour. Remove from oven and cool completely in the tin.
Make the icing. Mix icing sugar and cocoa, then add the butter and enough boiling water to create a smooth, spreadable icing.
Spread the icing evenly over the cooled caramel. Chill until firm, then lift from the tin and slice into squares to serve.