Preheat the oven to 180C/170C Fan/Gas 4 and line two baking trays with baking parchment.
In a large bowl, beat the butter and sugar together until pale and fluffy using a hand-held whisk or wooden spoon. Sift the flour, cornflour and spices into the bowl, add the salt and mix together until well combined. The mixture will look crumbly at this stage.
Tip the mixture out onto a lightly floured work surface and knead to form a soft dough. It might be difficult at first but keep going and it will come together.
Prepare two large pieces of baking parchment. Roll out the dough between the parchment to a thickness of 1cm/½in (the baking parchment will make it easier to roll).
Peel away the top layer of baking parchment and prick the dough all over using a fork. Cut the dough into Welsh dragon shapes using a biscuit cutter. Re-roll any leftover dough to cut out more shapes. (Only do this once to avoid over-working the dough.)
When you have used all the dough, put the shortbreads onto the prepared baking trays and bake for 18-20 minutes, or until just turning golden-brown at the edges.
Leave to cool on the baking trays for a few minutes, then lift the shortbreads onto a wire rack. Leave to cool completely before icing otherwise the icing will melt and run off.
For the icing, put the icing sugar in a bowl, add a third of the boiling water and stir together until thick. Leave it a minute then add more water, if needed to achieve a thick but spreadable icing. Spoon the icing into a piping bag fitted with a small plain nozzle.
First pipe a thin line of icing around the outside of the shortbread to create an outline in the shape of a dragon. Fill the outline with an even layer of icing, then gently shake the shortbread to smooth the surface of the icing and fill any gaps. Repeat until all the shortbread is iced.