Preheat your oven to 180℃ (160℃ if using a fan oven) and line a large tray or two with baking paper.
Add the softened butter to a large bowl with the caster sugar and brown sugar. Mix to make a thick paste, using an electric whisk or stand mixer to assist you if needed.
Next, add the egg and vanilla extract and mix well until smooth.
Stir in the self-raising flour then use your hands to press it into a ball of dough.
Set aside 12 Galaxy chunks (to top the cookies) and chop the rest of the chocolate into small pieces. Add the chopped pieces to the cookie dough and gently knead them in with your hands.
Divide the cookie dough into 12 pieces and roll each piece into a ball. Gently press down to create discs that are around 1 inch thick.
Top each cookie with a chunk of Galaxy chocolate and place them on the tray. I recommend only baking 4-6 per tray as they'll spread quite a lot in the oven.
Bake the cookies for 11-12 minutes until they're just starting to turn very slightly golden at the edges. They should look pale and wobbly in the centre- this is key to getting a soft and chewy texture! Do not keep baking them or they'll end up hard and crunchy.
Leave the cookies on the tray until they're completely cool before enjoying.