In a large mixing bowl, cream together the butter and sugar with an electric whisk until pale and fluffy. Crack in the egg, add the vanilla and whisk for another minute until combined. In a separate bowl sieve together the mixed spice, ginger, flour, baking powder and salt.
Add half of the dry mix into the butter mix and slowly combine adding the rest of the flour little by little. Just before the dough comes together add the pistachios, sour cherries and candied citrus to the mix in batches so that they are evenly combined throughout the dough.
Form the dough into a long sausage shape and roll it up in a sheet of baking paper. Place this in your freezer for two hours, it won't freeze fully but it will firm it up so that you can slice it. With a heavy knife, cut 2cm discs of the dough and place them on a lined baking tray. Bake the shortbread for 10 minutes at 180 degrees. Let them cool a little and eat them with a Christmas cocktail, a glass of port or a mug of something warm.