1. Line a 30cmx20cm shallow baking tin with baking paper. Preheat the oven to 160 degrees fan-forced or 180 degrees conventional.
2. In a large saucepan,melt the butter,add the spices,fresh ginger,sugar,treacle and golden syrup and warm through.
3. In a small bowl,dissolve the baking soda in a tablespoon of milk.
4. Crack the eggs into a bowl and whisk lightly. Whisk in butter and spice mix. Add baking soda and remaining milk,sift in the flour and cocoa and mix until smooth. Fold the chocolate chips through the batter.
5. Pour into the prepared tin and bake for 40 minutes or until cooked - the cake will be springy to the touch.
6. For the icing,in a bowl over just-simmering water,heat the butter. Add the cocoa powder and the ginger and lemon juices,then whisk in the icing sugar until dissolved and homogenous.
7. Allow the cake to cool then lift from the tray. Ice and allow the icing to set before serving.