Measure flour and baking soda in a sieve. Sift onto a piece of parchment paper or into a large bowl. Set aside.
In a separate mixing bowl, add in butter, caster sugar and light brown sugar. Using an electric mixer, cream the mixture for 3 minutes on medium speed, then scrape down the sides with a spatula. Cream the mixture for another 3 minutes on medium-high speed. The mixture should be of a pale brown colour, light and creamy. Scrape down the sides.
Add in the egg. Beat on medium speed for another 1 minute then scrape down the sides. Add in peanut butter and salt and cream for another 1 minute on medium speed.
Add in all the flour mixture. Beat on low speed until all the flour has just been incorporated. Add in the chocolate chips and beat on low speed until mixed into the batter. Use a spatula and scrape the sides and bottom of the mixing bowl so that the chocolate chips are evenly distributed. Cover the mixing bowl with plastic wrap and refrigerate the dough for at least 30 minutes, or up to overnight.
Preheat the oven to 160°C / 320°F Line baking sheets with parchment paper.
Scoop 1 tablespoon of dough. As the dough will be crumbly, squeeze to compact the dough together and roll to form a ball. Flatten the dough until it's about 1-cm thick and place on the lined baking sheet. Place the cookies 3-cm apart from one another on the baking sheet, as the cookies will spread during baking.
Bake for 15 to 17 minutes until golden on the edges. Let cool completely in the tray before storing in an airtight container.
Creamy peanut butter can be used instead. If you'd like to add chopped peanuts, add about 80 to 100 grams chopped peanuts.
To freeze, shape the cookies and place them on a baking sheet lined with parchment paper, making sure that they do not touch one another. Freeze until solid (about 1 to 2 hours) then transfer the dough into a ziploc bag. Bake the cookies while still frozen, for 17 to 20 minutes.