Place all the dry ingredients into a bowl and cut in the butter.
Rub between fingers until mixture resembles breadcrumbs, or pulse in a food processor.
Stir in the crystallised ginger.
Press all but 4 tbsp into an 8" (20cm) sq silicone mould or lined cake tin with the back of a spoon.
Sprinkle over the reserved mixture.
Bake at 180℃ (350℉, Gas 4) for about 25 minutes, when the top should be golden, but not burnt.
Cut the shortbread into 16 fingers whilst still warm, then leave in the mould to cool.
Adapted from The Great British Bake Off Big Book of Baking by Linda CollisterPlease note: calories and other nutritional information are per serving. They're approximate and will depend on exact ingredients used.