Mix the butter and sugar together at a medium speed until well blended. You don't want to beat too fast at this stage as unlike cakes, you don't want to add air or your biscuits will spread 

Again using a medium speed, add the salt and vanilla and mix in, then finally add in the flour. Stop mixing when your dough resembles breadcrumbs and tip out onto the counter

Using your hands, press and lightly kneed the dough until it comes together. Wrap your disc of dough in clingfilm and chill for 30 minutes until it has firmed up

On a lightly floured surface, roll the dough out until it is about 5mm thick. Using the smallest round cutter you have - mine was about an 2.5cm - cut as many circles as you can and place on a lined baking sheet

Place the baking sheet in the freezer while you heat the oven to 190°C / 375°F. Once the oven is to temperature, bake the biscuits for about 8-10 minutes or until the biscuits are cooked. Try not to let the edges colour

Place the biscuits on a cooking rack until full cooled. Meanwhile, make your royal icing

In a clean, grease free bowl, add the royal icing sugar. Add to tablespoons of water and using a hand held electric whisk, beat the water and royal icing sugar, gradually increasing the speed. Beat on the highest speed for about 3 minutes until the mixture is firm and thick and glossy

The consistency is key here. You are looking for something like toothpaste. Any looser and it wont keep its shape, any stiffer, and you will not be able to squeeze it out of the bag

Divide the mixture between two small bowls. Cover the surface of the icing in one of the bowls with cling film to prevent it drying out. Add a dab of gel colouring to one bowl until you get the colour you desire, then cover. Do the same with the other bowl

Take a disposable piping bag and place in a small star nozzle. Put one colour of the icing in the bag, and twist the top. Holding the nozzle a few mm above the biscuit, squeeze the icing until a star is formed almost the width of the biscuit, stop squeezing, then pull up

Once the first colour has run out, remove the nozzle from the bag and give it a quick rinse, then move to a clean bag. Fill with the second colour and continue piping stars until you either run out of icing or biscuits. 

Leave the icing to dry for about 8-10 hours (or longer if it is humid) then store in an airtight container and eat within a week