In a medium bowl stir together the sugar, cinnamon, cardamom, ginger, allspice and salt. Divide into two equal parts and set one aside. Add the flour and butter to the other half and stir until blended. It should be slightly grainy.
Press the dough evenly into an 20cm square pan. Cut into 4x8cm pieces using a knife and prick with the tines of a fork. This will keep the shortbread from warping while baking. Sprinkle the reserved sugar and spice liberally over the top, brushing into all of the cuts and holes.
Bake for 25 to 30 minutes in the preheated oven or until firm and golden at the edges. Do not brown. Cool completely in the pan and break into pieces along the lines to serve.