Line 2 x 12-section bun trays with 18 paper cake cases. Put the flour, ginger, baking powder and bicarbonate of soda in a bowl.
Put the butter in a small saucepan with the muscovado sugar and treacle and heat until the butter has melted and the sugar has dissolved. Remove from the heat and stir in the milk, then the egg. Add to the dry ingredients with the dried fruit and mix well. Divide the cake mixture between the paper cases.
Bake in a preheated oven, 160°C (325°F), Gas Mark 3, for 25 minutes or until risen and just firm to the touch. Transfer to a wire rack to cool.
Mix the icing sugar with the water to make a thick icing and drizzle over the cakes. Drizzle thin streaks of treacle over the icing.