Rub the butter and flour together in a bowl until you have a mix which is similar to breadcrumbs or use food processor. Stir in the sugar. Spread the mixture evenly into a 9" (23cm) square tin which has been greased. Bake in a pre-heated oven at 170C/340F (160C/320F if fan assisted) for approximately 35 minutes until it is golden brown. Allow the base to cool.
Heat the filling ingredients together in a pot with lid on, making sure that you check and stir it VERY regularly (otherwise it will stick!) as it simmers making sure it doesn’t simmer over. Continue stirring and checking until it thickens. Spread the filling evenly over the base and again allow to cool.
Melt the chocolate in a bowl over a simmering pan of water so that you can spread it over the filling.
When it has cooled, cut up into small squares with a sharp knife. Allow chocolate to soften slightly (maybe at room temperature for 15/20 mins) before or it will crack as you are cutting it, then you can put back in fridge!