Preheat oven to 180 C / Gas 4. Grease and flour a 26cm round tube tin.
In a large bowl, cream butter and sugar until light and fluffy. Add eggs one at a time, beating well with each addition. Stir in courgette and 75ml rum.
In a separate bowl, mix flour with nuts, chocolate, cocoa, baking powder, bicarbonate of soda, salt and cinnamon. Stir flour mixture and milk into egg mixture until well blended.
Pour batter into prepared tin. Bake at 180 C / Gas 4 for 50 to 55 minutes or until skewer inserted into centre of cake comes out clean. Let cake cool in tin for 15 minutes, then invert onto wire rack and cool completely.
Meanwhile, make the rum glaze. In a medium bowl, combine icing sugar with 3 tablespoons rum. Mix until smooth. Drizzle over the cold cake.