Add the butter and the sugars into the mixing bowl, mix thoroughly until the butter and sugar are well combined and creamy.
4.4 ounces butter, 3.5 ounces light brown sugar, 2.6 ounces caster sugar
Add the egg and the vanilla extract and mix to combine. You don't need to beat the egg into the butter/sugar mix as we're not trying to add air.
1 egg, 1 teaspoon vanilla extract
Add in the flour and the baking powder. Mix once more until everything is well combined and there is no flour hanging around the sides of the bowl.
5.3 ounces plain flour, 1 teaspoon baking powder
Mix in the rolled oats and stir until they are completely mixed in.
5.3 ounces rolled oats
Add the chocolate chips and stir until they are evenly mixed though the dough. It may be getting a little bit stiff but just keep stirring it will all come together.
5.3 ounces chocolate chips
Line the baking tray with parchment/greaseproof paper and use a tablespoon or cookie scoop to portion out pieces of dough. Scoop two tablespoon amounts of dough per cookie out onto the baking tray. Mould them into a roughly spherical shape with your fingers but don't compress them. Depress the top slightly to give a circular cookie.
If you weigh out 100gram amount of the dough you will make about 8 large cookies - bakery style.
Place this tray into the refrigerator and allow it cool for approximately 30 minutes. (Follow this step if you want a nice thick cookie.)
Preheat the oven to 400F/200C/180C Fan
Cook for approximately 12 minutes until the edges are slightly golden and the tops are still a little soft. They'll firm up as they cool. (Bake for 10 minutes for a soft-bake cookie and 14 minutes for a firmer more biscuit-like cookie.) They're done when they are golden brown in colour.
Leave them on the baking tray for a few minutes before moving them onto a cooling rack and allow them to cool. They're a little fragile straight out of the oven. Resist munching them straight away as the chocolate will be quite hot from the oven.