Grease and line 2 large baking trays. Whisk the butter and caster sugar in a bowl until pale and creamy. Whisk in the egg yolk and ½ tsp vanilla extract.
Sieve in the flour and baking powder, and combine until the mixture resembles a crumble. Using your hands, bring the mixture together to form a firm dough. On a lightly floured surface knead the dough until smooth and pliable.
Roll the dough out to a thickness of 5mm and using a 6cm petal cutter stamp out cookies. Prick with a fork and chill for 1 hr on the baking tray.
At this stage you can either wrap and freeze the remaining dough for up to 6 months or roll it out as above and make a further 20 cookies.
Ten minutes before baking, preheat the oven to 180°C (160°C fan, 350°F, Gas 4). Bake the cookies for 10-12 minutes until firm and lightly golden. Leave to cool for 10 minutes before transferring to a wire rack.
Once cool, the cookies are ready to ice. Sift the icing sugar into a bowl, adding the remaining vanilla extract and the egg whites. Mix until combined.
Combine the wet and dry ingredients to make a soft, spreadable icing – if it’s too stiff add a few drops of water, or if too dry, add some more sifted icing sugar, 1 tbsp at a time. Beat until smooth and glossy.
Divide the icing between 3 bowls, leaving one batch white. Colour one with a few drops of red colouring and the other with a few drops of yellow colouring.
Working with one batch at a time, spoon 2 tbsp icing into a small piping bag fitted with a No: 2 size plain piping nozzle. Pipe a line approx. 1mm from the edge of one third of the batch.
Spoon ¾ tsp of the icing into the centre and spread, tapping gently to make an even layer and release any air bubbles.
After 1 hr the icing will be firm enough to pipe on if you would like to add any extra detail to your cookies. To finish, use a small blob of any remaining icing to secure a wafer daisy in the centre of each cookie. Leave for a few minutes until set before serving.