To make the base, beat the Stork and sugar until light and fluffy, sift the flour over and fold in. Press the mixture evenly over the base of a greased 18 x 28cm (7 x 11 inch) Swiss roll tin.
Bake in the centre of a preheated oven at 180ºC, 350ºF, Gas 4 for 15-20 minutes, until golden brown.
To make the filling, place all the ingredients, except the hazelnuts, in a heavy-base saucepan and simmer gently for 8-10 minutes, stirring continuously.
Spread the filling over the cooked base and allow to cool. When cool, scatter the hazelnuts over and press down slightly into the filling.
Break the chocolate into a bowl and melt slowly over a pan of hot water (or in a microwave on medium power). Quickly pour the melted chocolate over the caramel and swirl with a fork.
Mark into portions before the chocolate sets then leave to cool completely.