Preheat the oven to 180°C (350°F/Gas 4). Lightly grease an 18 × 28cm (7 × 11¼inch) shallow cake tin and line with baking paper, leaving the paper hanging over on the two long sides.
Sift the flour into a bowl, then mix in the coconut and sugar. Add the melted butter and stir thoroughly. Press the mixture firmly into the tin and bake for 12–15 minutes, or until lightly golden. Allow to cool.
To make the caramel filling, place all the ingredients in a small saucepan over low heat. Slowly bring to the boil, stirring constantly, then boil gently, stirring, for 4–5 minutes, or until lightly caramelised. Quickly pour over the cooled base, spreading evenly. Bake for 10 minutes, then set aside to cool.
To make the chocolate topping, place the dark chocolate and Copha in a heatproof bowl. Half-fill a saucepan with water and bring to the boil. Remove the pan from the heat. Sit the bowl over the saucepan of water, making sure the base of the bowl does not touch the water. Stir occasionally until the chocolate and Copha have melted. Spread the chocolate topping over the cooled caramel, then refrigerate for 20 minutes, or until set. Lift the slice from the tin, using the paper as handles. To serve, cut into pieces with a hot, dry knife.