Preheat the oven to 180°C/fan oven 160°C/350°F/Gas mark 4.
In a mixing bowl, add the butter and sugar and beat together until light and fluffy in appearance. Add the eggs, one at a time, and beat well until fully incorporated. Add the orange zest, flour and milk and fold in gently with a spatula or large metal spoon. Turn into the prepared tin (I used a loaf pan that was a little big, so it came out pretty flat looking), smooth the top of the mixture and bake in the oven on middle shelf 35 -45 minutes or until a skewer inserted into the centre comes out clean. Remove from the oven and leave to cool in the tin. When cool make little holes in the cake with a skewer, (this is important for pouring the syrup onto the cake to ensure the syrup soaks in fully.)
For The Orange Syrup
Put the orange juice and granulated sugar into a pan and heat gently until the sugar has dissolved. Bring to the boil and boil for a couple of minutes. Pour over the top of the cake. When all the juice has soaked in, carefully remove the cake from the tin.
For The Topping
Melt the chocolate by placing a heatproof bowl over a saucepan of barely simmering water. Once the chocolate has melted, pour over the top of the cake. Either smooth the top over with a spatula or make a little pattern with the prongs of a fork. Add sprinkles if desired, leave chocolate to set before cutting into the cake.