Place the chocolate in a heat-proof bowl, and place over a saucepan of simmering water to melt, careful not to get any steam or moisture onto the chocolate or it will split. Once melted, set aside to cool.
Using an electric mixer, beat the butter and sugar until well combined. Add the eggs, one at a time, beating well after each addition. Add the vanilla, melted chocolate and milk and beat on low for 15 seconds until combined.
Sift the flour, cocoa powder, baking powder, allspice, cinnamon, ginger and a pinch of salt into the butter mixture and use a wooden spoon to combine well. Stir in the walnuts and refrigerate for at least 3 hours, or overnight, until the mixture is firm.
Preheat the oven to 180ºC. Line a couple of large baking trays with baking paper.
Sift the icing sugar into a bowl. Roll tablespoonfuls of the cookie mixture into balls and roll each ball in the icing sugar to coat. Place each ball onto the baking trays, spacing them in an offset alignment. This allows for air circulation and leaves room for the cookies to spread slightly.
Bake for 18-20 minutes or until just firm to touch. Remove from the oven and allow to cool on the tray for 3-4 minutes. Transfer to a wire rack to cool completely.