Preheat your oven to 180°C (or 160°C if using a fan oven) and line a large baking tray or two with baking paper.
Add the butter, caster sugar and light brown sugar to a large bowl. Mix well to make a thick paste, using a stand mixer or electric whisk if needed.
Next, stir in the egg and vanilla extract.
Stir in all of the flour and use your hands to press it into a ball of dough. If the dough is too sticky, add more self-raising flour. If it’s too dry, add a splash of milk to help it stick together.
Add the chocolate chips and Daim bar pieces and gently knead them into the dough using your hands.
Divide the dough into 12 equal-sized pieces and roll each piece into a ball.
Place them onto your lined tray, leaving a couple of inches between each to stop them from sticking together while baking. Around 4-5 per tray works best.
Bake for 11-12 minutes until they are JUST beginning to turn golden brown at the edges. They will look quite underdone at first but they will turn golden and chewy once they've fully cooled.
Leave the cookies on the tray until they're completely cool, then enjoy!