Preheat your oven to 180°C (160°C for fan ovens) and line two large trays with baking paper.
In a large bowl, mix together the butter, caster sugar and brown sugar until soft and fluffy. Then stir in the egg and the vanilla extract.
Mix in the flour and cocoa powder, then use your hands to press everything together into a ball of dough.
Pour the white chocolate chips into the bowl and use your hands to press them into the dough.
Divide the dough into 12 equal pieces and roll each piece into a ball.
Use your thumb to make a small dent in each ball, place a fun-size Milky Way bar in there, then wrap the dough around to cover it completely. Reroll to make a perfectly round ball.
Place the balls onto your lined trays, leaving a couple of inches in between to allow for spreading.
Bake one tray at a time, for 11-12 minutes (they'll look a little underbaked at first but will firm up as they cool).
While the cookies are warm, gently place the Magic Stars and extra chocolate chips on top.
Leave the cookies on the tray until completely cool then enjoy!