Pre-heat the oven to Gas Mark 5/190C/375F. Line a baking tray with greaseproof paper.
Weigh out the peanut butter, caster sugar and baking powder in a bowl Cream together with a wooden spoon. (A whisk will get clogged up very quickly!).
Add the egg and chocolate chips and mix thoroughly. If you are using crunchy peanut butter, the batter will appear lumpy. The key thing is to make sure it's all mixed together properly.
Dollop the mixture onto the baking tray with a teaspoon. Keep the dollops at least 5cm away from each other. The cookies will spread out flat as they heat up and will join together if there's not enough space between them.
Bake in the top of the oven for 5-10 minutes, until the sides of the cookies are starting to turn brown. The middle of the cookies will appear liquid/wobbly when you take them out of the oven. Leave them to cool on the baking tray, as the centres will harden up a bit. Don't attempt to move them when they are hot, as they will break up.