Cream together the butter and sugar until light and fluffy. Add the flour and stir until the ingredients are thoroughly mixed. Knead until smooth.
Press the dough evenly into a greased deep square or rectangular tin lined with baking parchment and prick the surface with a fork.
Bake in the preheated oven for 25 to 30 minutes. Remove from the oven and leave to cool in the tin.
For the filling:
Meanwhile, place the filling ingredients into a saucepan and stir on a low heat until the sugar has dissolved. Bring to the boil slowly, then cook, stirring continuously for 5 to 10 minutes. Cool slightly, then pour over the biscuit base and leave to set.
For the topping:
For the topping, melt the dark chocolate in a bowl over a pan of simmering water and spread over the caramel. Leave in the fridge to set then cut into squares with a sharp knife and serve.