1. Sift the flour into a bowl and add the butter. Use your fingertips to rub the butter into the flour. Stir in the sugar and vanilla extract and oats. Bring the mixture together to form large clusters using a palette knife, adding the milk as necessary until you have a soft dough. Wrap in cling film and chill for 30 minutes.
2. Preheat the oven to 180C/160Fan/Gas 4. Line 2 baking sheets with baking paper.
3. On a flat surface lightly dusted with flour, roll out the dough to a thickness of approx. 1cm. Stamp out 8 x 7cm round cookies, re-rolling the trimmings.
4. Bake for 12-15 mins or until pale golden. Cool on the baking sheet for 10 minutes then transfer to a wire rack and leave until cool.
5. Meanwhile, weigh and divide the royal icing sugar equally between 5 small bowls. Add colouring paste to each bowl to make a very dark blue, a lighter blue, purple and dark pink; leave one white, mix with a little water (approx. 1 - 4tsp) until smooth. Spoon into separate disposable icing bags. Snip off the end in order to pipe lines approx. 4mm width.
6. Pipe circles of each colour on top of the cookies, starting on the outside with the darkest colour and finishing in the middle with white. Use a round bladed knife to blend some of the icing together. Use the end of a cocktail stick to ‘feather’ the icing into swirling patterns. Sprinkle the icing with edible glitter. Leave icing to dry before serving
Food colouring pastes are concentrated and therefore ideal for adding to icing to make strong colours. You can find them in some large supermarkets as well from online cake decorating suppliers and stores like Lakeland and Hobby Craft.
Alternatively you will find gel food colours in all major supermarkets which are fine to use but won’t provide quite the same depth of colour.