Pre-heat oven to 180 C / Gas 4. Grease and line a medium-sized cake tin or loaf tin.
Cream the butter or margarine and sugar together well. Add the eggs, chocolate chunks and grated orange zest, and mix well.
Sift the flour and fold it in gently, making sure it is well-mixed in, then add the milk and mix well. Spoon the mixture into the cake tin.
Bake in the pre-heated oven for 45 minutes, or until golden brown and a skewer inserted into the middle comes out clean.
To make the syrup: Mix the orange juice and icing sugar together well and heat in a small saucepan, until the sugar is dissolved.
Remove cake from oven and immediately make lots of holes in it, all over, with a skewer, then pour the the orange-sugar syrup over it. Leave in tin to cool completely before removing.
When cake is completely cooled, melt the chocolate (either in a bowl set over a pan of warm water, or carefully in a microwave on a low heat). Put into a piping bag and drizzle over the cake. Leave chocolate to set before slicing the cake.