Pre-heat oven to 150 degrees/gas mark Line two baking trays with greaseproof paper.
Put the sugar; butter; golden syrup; egg and vanilla extract into a large mixing bowl. Mix with an electric mixer on low speed for 3 minutes, until there are only small lumps of butter visible, then turn the speed of the mixer up to medium for 1 minute to reduce the size of the butter lumps.
Sift both types of flour into the bowl and mix on a low speed for 1 minute, or until combined, then turn the mixer up to medium speed for 30 seconds, or until the dough clings together and the sides of the bowl are clean.
Turn the dough out onto a clean work surface. Divide into two and squash into even flat discs. Cover with cling film and chill or use immediately. Roll out the dough to an even thickness between two sheets of greaseproof paper and cut out your biscuit shapes
Carefully pick up each biscuit with a palette knife and place onto the lined trays. Space the biscuits out evenly – they will spread a little when baking.
Place the trays into the preheated oven and back for 20 – 30 minutes (check at 20 minutes).
When the biscuits are evenly cooked and just beginning to turn a golden colour, remove from the oven and transfer the whole sheet of greaseproof paper to a cooling rack. Do this very carefully, as the biscuits will be fragile and hot. Allow to cool completely before icing.
Combine the ingredients in a mixing bowl, adding the liquids first - add 100ml water to the bowl to start with or most of the fresh egg whites. Add the dry ingredients and whisk for 5 minutes with an electric whisk, or for longer with a wooden spoon. Whisk slowly initially to avoid clouds of icing sugar! Continue whisking, gradually adding more water (or egg white), until you achieve the desired consistency - a smooth, bright white paste, which is the thickness of toothpaste (you may need to use less or more water than the recipe states - go slowly and judge when you have reached the correct point).
There are two different uses for royal icing; Line icing and Flood icing. Two-thirds of the royal icing mixture is for making flood icing and the other third is for line icing.
Line icing: No need to change the consistency of the royal icing recipe for line icing as it needs to be like toothpaste. Simply add your food colouring, a few drops at a time until you have the desired colour you want and start doing your lines.
Flood icing: Place the royal icing in a large bowl. Add water, a few drops at a time, stirring constantly, until you have a pourable mixture of the same consistency as custard. Then add your food colouring and start filling in the middle of the biscuits.