Preheat oven to 180 degrees Celsius Line 2 baking trays with non-stick baking paper. Sift flour, cocoa, spices and baking powder together into a bowl.
Beat butter with electric beaters until smooth and creamy. Add brown sugar and golden syrup and beat until well combined. Add egg and beat until smooth.
Add dry ingredients and mix to a stiff dough. Turn out into a lightly floured surface and knead lightly until smooth. Pinch off walnut sized pieces of dough and roll into balls.
Arrange onto baking trays, leaving a little room between each for spreading. Bake for 15 minutes. Cool for 5 minutes then transfer to wire racks to cool completely. Store in an airtight container for 2 weeks.
To decorate, sift icing sugar into a small bowl. Add butter and milk and stir until spreadable. transfer to a piping bag or a snap-lock bag. Snip the corner and drizzle across biscuits.