Cream the butter and sugar together in a bowl until light and smooth. Stir in the egg with 1 tbsp flour. Stir in the remaining flour, baking powder, mixed spice and ginger until the mixture comes together as a dough. Wrap in clingfilm and chill for 30 minutes.
Preheat the oven to 180°C/fan 160°C/gas mark 4. On a floured surface, roll out the dough to the thickness of a pound coin. Stamp out biscuits using a cookie cutter and place on to baking sheets in batches. Bake for 10-12 minutes, until just golden. Transfer to a wire rack and leave to cool completely.
Mix the icing sugar with 1½ tbsp water. Spoon into a piping bag and ice the cookies with patterns of your choice.