Grease a 2lb loaf tin and pre-heat oven to 170C or 160C if a fan.
Put the butter and yacon in a small saucepan and melt over a low heat. Stir occasionally and then remove from the heat.
Sift the flours together with the baking soda and the spices in a mixing bowl. Add the coconut sugar and salt, stirring it all together. Add the egg and milk and mix until smooth. Finally, gradually add the melted butter and yacon mix, stirring well until all combined.
Pour the batter into the greased tin and bake for about 30 mins, if in the fan oven, slightly longer if not, until risen and firm to touch. Allow to cool in the tin for 5 mins before turning out onto a wire rack to cool completely.
Warm a tablespoon of coconut sugar in a saucepan over a low heat with a little water until dissolved. Then zest in some fresh ginger and drizzle over the top of the cooled loaf.