1. Preheat the oven to 180°C, gas mark 4 and grease an 18 x 28cm Swiss roll tin.
2. To make the base, cream the butter and sugar together in a mixing bowl until light and fluffy. Fold the four into the creamed mixture until well mixed. Press the mixture evenly into the base of the prepared tin and bake in the centre of the preheated oven for about 12 minutes until golden.
3. Meanwhile, make the filling Place all of the filling ingredients into a saucepan, bring to a gentle boil, then continue to simmer for 5-10 minutes, stirring continuously. You need to stir quite vigorously as little bits of caramel will stick to the pan but should come away easily and dissolve. Once the sugar has dissolved and the mixture has turned a light caramel colour, take the pan off the heat and spread the filling all over the cooked base. Leave to cool.
4. Melt the chocolate in a bowl set over a saucepan of simmering water, making sure the base of the bowl does not touch the water. Stir until the chocolate is completely melted and the pour it over the caramel. Leave to set for 2 hours or until the chocolate is firm. Cut into 12 fingers.