Preheat oven to a 180 C and line a large rectangular (20 x 30 cm) baking tin or oven dish with baking paper.
Sift all dry ingredients together. Mix the liquids together and gradually fold into the dry ingredients. Pour batter into tin or dish.
Bake for 20-25 minutes or until a skewer comes out clean. Remove from the tin and cool.
Topping: Meanwhile, place all the ingredients, except the chocolate, in a pot and stir over a low heat to dissolve the sugar.
Bring to the boil and simmer gently. Stir until the mixture thickens. Pour over the cooled cake.
Sprinkle with grated chocolate.
Citrus and spice cake: Replace the cocoa in the cake with more cake flour and add 1 tsp grated lemon or orange rind. Use 3 tbsp lemon or orange juice instead of the vinegar. Add 2 tsp ground mixed spice or cinnamon to the batter. Bake as above and dust with a little icing sugar.
Cupcakes: Halve the recipe above. Line a 12-hole cupcake pan with paper cups. Spoon cake batter into the cups and bake for 15-20 minutes or until a skewer comes out clean. Make only half the topping.
Dust cake with icing sugar or cocoa instead of the topping, if preferred.