My grandmother's recipe
Beat together 4oz of the margarine with the castor sugar. Sift the flour in when fluffy. Knead together. Press into a long, shallow baking tray (7" × 12"). Bake on the middle shelf at 180C (350F) for twenty minutes.
Meanwhile, place the other 4oz of the margarine, the condensed milk and the brown sugar in a saucepan. Boil for five mnutes, stirring contnually. Add a few drops of vanilla essence and beat. Pour over the base and allow to cool.
When cool, melt the chocolate and pour over the top. Wait till this has set and then cut it into fingers