Preheat your oven to 180℃ (160℃ for fan ovens) and line two large trays with baking paper.

Add the butter, caster sugar and brown sugar to a large bowl. Use a stand mixer or electric whisk to mix them together to create a thick paste.

Next, add the egg and vanilla extract. Mix well until smooth.

Stir in all of the self-raising flour. When it's starting to stick together, use your hands to press it into a ball of dough.

Add the Skittles to the cookie dough, leaving a handful aside to top the cookies with at the end.

Divide the dough into 12 equal-sized pieces and roll into balls. Lightly press on them with your fingers to make discs that are around 1 inch thick.

Place the cookies onto your lined tray, leaving a couple of inches of space in between to allow for spreading. I recommend baking 4-6 per tray.

Bake for 11-12 minutes until they are JUST starting to turn golden brown at the edges. They'll look underdone at first but will continue browning and firming up as they cool.

While the cookies are still warm and gooey, gently press a few of the leftover Skittles onto each one. Doing this after baking rather than before prevents the Skittles from cracking or melting.

Let them sit on the tray until completely cool before enjoying.