This was my Mum's signature slice when we were growing up. Great ginger crunch is all about the icing to base ratio. Two thirds base to one third icing is the money shot. Be careful to get the consistency of the icing right here, you don't want it too wet.
Step 1: Base
Preheat oven to 180 degrees.
Grease and line a 25cm x 20cm baking tin.
Cream the butter and sugar until pale. Sift in the dry ingredients and mix until combined. Press evenly into the lined tin. Bake for 20-25 minutes until the base is firm to touch. Remove from oven, smooth out the edges with the back of a spoon, and keep warm while you get the icing ready.
Step 2: Icing
Heat all the ingredients in a small saucepan. Stir until melted and combined. Pour the icing over the warm base and spread out even. I like to cut the ginger crunch while in the baking tin, just before the icing is fully set. Use a warm wet knife to do this.