Preheat the oven to 190 degrees C. Grease a smallish shallow sponge tin, about 30 x 18cm, and line with nonstick baking paper (make a case by clipping the edges of the paper together with metal paperclips).
Sift the dry ingredients together. Cream the butter and sugar thoroughly with an electric mixer until light and pale lemon in colour. Add the dry ingredients and blend together with a wooden spoon. Turn the mixture onto a clean surface and knead until no longer crumbly. Put the mixture in the prepared tin, pressing it right into the corners.
Bake for about 15 minutes, or until a light golden brown (watch it doesn't burn) and leave it in the tin for 10 minutes. Cut into squares while soft, but leave it assembled in the tin. Lift the paper case out of the tin and transfer the ginger crunch, with paper still attached, to a wire rack.
To make the icing, combine all the ingredients in a small saucepan over low heat. Pour the icing over the cooled ginger crunch, spreading it evenly with a spatula. Leave until the icing is quite tacky, then cut through the icing and separate the squares carefully.
Remove the ginger crunch squares from the paper and cool on a cake rack until the icing is set. If the icing won't set, refrigerate the ginger crunch for 15 minutes.
Ginger–Lemon or Ginger–Orange Crunch: Add the grated zest of 1 lemon or 1 orange to the slice mixture. Sprinkle the icing, before it sets, with chopped glacé lemon or orange peel.