Preheat the oven to 180°C (Gas mark 4/350°F). Line two large baking trays with grease-proof or baking paper
Whisk 150g butter, 125g caster sugar and the egg yolk together in a large bowl until fluffy.
Add the vanilla extract and yellow food colouring (if using) and whisk until well combined.
Add the self-raising flour, whisk until just combined, then pull together with your hands to form a dough.
Split the cookie dough mixture into 18 roughly equal pieces, (each around one tablespoon in size), then roll each piece into a ball.
Pop 2 Tbsp caster sugar into a small bowl. Roll the cookie balls around in the sugar to coat all over and place on the baking tray, leaving plenty of space in between each one for spreading (these cookies spread out a lot).
Bake the cookies for 10-12 minutes, then remove from the oven.
While the cookies are baking, carefully cut each of your orange or pink smarties in half for chick beaks.
As soon as the cookies are cool enough to handle, but still warm, carefully push 2 candy eyes and one half smartie beak into the cookie to make a chick face.
Leave the cookies on the tray for 10 minutes before carefully transferring to a cooling rack to finish cooling.
Once completely cool, these cookies can be stored in an airtight container for up to 3 days.