Soften the butter at room temperature until it can be easily pressed by fingers, then add brown sugar and white sugar, and stir evenly with a spatula until there are no obvious sugar granules.
Add the egg mixture and vanilla extract and stir with a spatula until the butter fully absorbs the egg mixture. p.s. I find it hard to mix everything with a spatula at this step. I used a hand mixer.
Add cake flour, baking powder and salt and stir evenly with a spatula. p.s. "Cake flour" seems to be a name more widely used in the US. It's called low-gluten flour in China. It contains low protein (around 8%). In contrast, plain flour contains medium protein (usually 10-12%). It's not easy to get cake flour in the UK. I often mix 4 portions of plain flour (the most basic supermarket versions, which usually contain relatively low protein, around 10%) and 1 portion of cornflour (contains almost no protein) to get 5 portions of dupe cake flour. It works fine.
Finally, add the chocolate chips and stir evenly with a spatula.
After forming the dough, cover the bowl with cling film and refrigerate for 1 hour. ps. I put it into the freezer for a shorter time.
Take the cookie dough out from the refrigerator, divide it into 10 portions (around 50g each) and roll into balls.
Use crunchy peanut butter as the filling. If it's cold, just put a spoonful of peanut butter into the cookies (in the next step). If the weather is hot, take a baking tray, line it with baking paper, spread spoonfuls of peanut butter onto the paper, and refrigerate for an hour. p.s. I didn't refrigerate the peanut butter. As long as the peanut butter you use is not too runny, it's not necessary to refrigerate. I find it easy to put 10g into each cookie; 12g is also not too hard; but 14g may be too much.
First take a cookie dough, press it into a small bowl shape with your thumbs, and put a spoonful of peanut butter into it.
Then seal the dough tightly and place it on the baking tray with the seam facing down.
Repeat the previous two steps to get 10 cookie dough balls.
Then flatten each cookie dough gently. p.s. Don't flatten too much.
Preheat the fan oven to 170 degrees celsius and bake for 15 minutes. p.s. I baked at 180 degrees for 15 mins in my Bosch oven using the air mode. Need to adjust the temperature and time if you are using a conventional oven.
After baking, place it on a cooling rack to cool, then store it in a sealed box.