Preheat oven to 180 degrees Celsius (160 degrees fan-forced).
Line one to three large baking trays with non-stick baking paper. If you have a fan-forced oven, you can use three trays and bake all of the cookies at once. Otherwise, bake the cookies one or two trays at a time so they bake evenly.
Using an electric mixer or electric hand-held beaters, beat butter, caster sugar, brown sugar and vanilla together until pale and creamy.
Stop the machine occasionally and use a spatula to scrape down the side and base of the bowl. Add the egg and beat until well combined.
Stir the plain flour and self-raising flour together in a small bowl. Add the flour and chocolate chips to the butter mixture and stir until well combined.
Form heaped teaspoons of dough into balls and place balls about 6cm apart on a baking tray. If baking the cookies in batches, you can refrigerate the dough between batches to prevent it from becoming too soft.
Bake for about 13-15 minutes, or until cookies are lightly golden but still very soft (they will firm up on cooling). The longer the baking time the crunchier the cookies will be. If you are using two trays, and you don't have a fan-forced oven, swap the trays around halfway through the baking time so that the cookies brown evenly.
Leave cookies to cool on the tray for about three minutes, and then transfer them to a wire rack to cool completely.