In a mixing bowl, sift the flour, spices and raising agents together. Cream the butter and sugar together in a separate large bowl. Blend the egg yolk together with the black treacle, vanilla extract and milk. Whisk into the creamed mixture.
Add the flour to the batter and mix until the dough just comes together. Wrap the dough in cling film and chill for 30 minutes to rest.
Preheat the oven to 200°C/Fan 180°C/Gas 6 and line a baking sheet with non-stick baking paper. Lightly knead the dough and roll out on a lightly floured work surface.
Stamp out cookie shapes and place on the prepared baking sheet, spaced apart.
Bake for 8 minutes until golden. Transfer to a wire rack to cool.
To make the royal icing, lightly whisk the egg white in a large, clean bowl until frothy. Add the icing sugar and beat until the icing resembles freshly whipped double cream.
Add the lemon juice through a tea strainer and continue beating until smooth and glossy. The icing should be mallowy and not too stiff.
Aim for the consistency of freshly whipped double cream. If it looks grainy, add a little more egg white or cool water.
To decorate the biscuits, fit a piping bag with a No. 2 nozzle and fill with the white royal icing. Decorate the biscuits and then leave until set before storing in an airtight container. Store at an airtight container at room temperature for up to a week.