Preheat oven to 150°C (fan). Grease and line a loaf tin (21 x 10cm, 7cm depth).
In a saucepan over low heat, melt together the grated ginger, treacle, golden syrup, dark brown sugar, and butter until smooth.
Remove from heat and stir in the boiling water. Allow to cool slightly.
In a large bowl, whisk together the dry ingredients (flour, ground ginger, cinnamon, nutmeg, and salt).
In a separate small bowl, beat the eggs with the milk.
Stir the bicarbonate of soda into the cooled syrup mixture, then pour into the dry ingredients, followed by the egg mixture.
Whisk until smooth, then fold in the chopped stem ginger.
Pour the batter into the prepared tin and bake for 55-65 minutes, or until a skewer inserted in the centre comes out clean.
Prepare rum syrup by combining water, rum, golden caster sugar, sliced ginger, and salt.
Make icing by mixing icing sugar, ground ginger, and milk until desired consistency.
Drizzle icing over cooled cake.
