Preheat your oven to 180°C (160°C for fan ovens) and line a large tray or two with baking paper.
Start by adding the softened butter to a bowl with the caster sugar and brown sugar. Mix to make a thick paste, using an electric whisk or stand mixer to assist if required.
Add the egg and vanilla extract. Mix well until smooth.
Add all of the self-raising flour and mix it in. When it starts to come together, use your hands to press it into a ball of dough.
Add the Milky Bar chocolate and gently knead it in with your hands.
Divide the dough into 12 equal-sized pieces and roll into balls. Lightly press them down until around 1-inch thick.
Place the cookies on the tray, leaving a couple of inches between them to allow for spreading. Around 4-5 per tray works well.
Place 1 chunk of Milky Bar chocolate onto the centre of each cookie.
Bake for 11-12 minutes until they're JUST turning golden brown at the edges. They will look gooey and underbaked but resist the urge to keep baking- they will firm up as they cool.
Let the cookies cool completely before removing them from the tray.