1 Preheat the oven to 160C/325F/gas mark 3.
2 Sift the flour, cornflour, sugar, ground ginger, baking powder and salt into a bowl, then add the diced butter. Mix together with a wooden spoon or, using an electric mixer, mix on medium speed with the paddle attachment until everything comes together.
3 Press the dough evenly into an 11 x 34cm baking tin lined with baking parchment. Place in the oven and bake for 20 minutes.
4 About 5 minutes before the shortbread base is cooked, make the icing: combine all the ingredients in a small pan and heat gently, stirring constantly until the butter has melted.
5 Pour the hot icing over the shortbread base as soon as it comes out of the oven.
6 Cut into slices while still warm, then leave to cool.