1.Preheat oven to 160°C (140°C fan-forced). Grease a 15cm x 23.5cm x 9cm (6-inch x 9½-inch x 3¾-inch) loaf pan; line base and sides with baking paper, extending the paper 5cm (2 inches) over the sides.
2.Stir butter, sugar, treacle, syrup and milk in a large saucepan over medium heat until butter melts and sugar dissolves. Bring to the boil (mixture may look slightly curdled). Remove from heat; stir in soda.
3.Sift flour and ground spices into a large bowl; stir in butter mixture and crystallised ginger until just combined. Stir in egg until just combined. Pour mixture into pan; cover with a piece of pleated foil (see note).
4.Bake loaf for 1 hour. Remove foil; bake a further 30 minutes or until a skewer inserted into the centre comes out with a few moist crumbs attached (the cake will have a crack along the top). Cool loaf in pan.
5.To make ginger ale icing; bring ginger ale to the boil in a small saucepan over medium heat; boil 6 minutes or until reduced to 2 tablespoons. Remove from heat; stir in butter until melted. Transfer mixture to a small bowl; whisk in sifted icing sugar and 2 teaspoons of the juice until combined. Gradually stir in enough of the remaining juice, if needed, until icing is pourable.
6.Pour immediately over cooled cake. Stand cake until icing sets.
7.Serve loaf dusted with extra ground ginger.
Pleating the foil allows the cake to expand as it cooks. Use a piece of foil slightly larger than the pan. Fold the foil to create a pleat down the centre. We used a loaf pan with a capacity of 3 litres (12 cups). The uniced cake can be made a day ahead; store in an airtight container at room temperature. The uniced cake is also suitable to freeze.