Preheat your oven to 180℃ (or 160℃ for fan ovens) and line a large baking tray or two with baking paper.
Add the butter to a large bowl along with the caster sugar and brown sugar. Use an electric whisk or stand mixer to mix it into a thick paste.
Now add the egg and the vanilla extract. Mix again until smooth.
Add in all of the self-raising flour and stir it in. When it's starting to stick, use your hands to press it all together into a ball of dough.
Gently knead the chocolate chips and Crunchie chunks into the cookie dough.
Divide the dough into 12 equal-sized pieces and roll them into balls. Gently press on them with your fingers to make discs that are roughly 1 inch thick.
Put the cookies on the tray, leaving 2-3 inches of space in between them to allow for spreading.
Bake for 11-12 minutes, until the cookies are JUST starting to turn golden at the edges. They'll look a bit underdone at first but don't keep baking them! They will turn golden and chewy once they've cooled down.
Let the cookies cool completely before removing them from the tray.