Preheat oven to 180ºC. Line a 20cm (base) deep square cake tin with baking paper. Beat
butter and sugar in the small bowl of an electric mixer until pale and creamy. Add treacle and
beat until just combined. Add eggs, 1 at a time, beating after each addition.
Transfer mixture to a large bowl and stir in carrot and ginger. Sift flours, cinnamon, ground ginger and bicarb soda into a small bowl, then add to carrot mixture. Add milk and stir until just combined. Spoon mixture into prepared tin and bake for 45-50 minutes or until a skewer inserted into the centre comes out clean. Stand tin on a wire rack for 10 minutes before turning out to cool.
Sift icing sugar into a medium bowl, make a well in the centre, add extra butter and lemon juice, then beat with a wooden spoon until thick and creamy. Spread icing onto cooled cake and sprinkle over extra cinnamon. Serve.