Using a paddle attachment or normal beaters, cream the butter. Gradually beat in the sugars until fluffy (it won't be light because of the brown sugar).
Add the egg, peanut butter, vanilla essence, salt and baking soda and beat well until it forms a sticky, slightly runny goo.
Add the flour and mix well until the mixture is all combined and forms a dough. You may need to use your hands near the end.
Take heaped teaspoons of the dough (about 2-3 cm or 1 inch in diameter) and roll into a ball. Place on the tray and press down with a fork.
Bake for 15 minutes or until the desired colour is reached.
Use whatever peanut butter you like. It does not matter if it is crunchy or smooth. If you are using salt-reduced peanut butter (and salt-free butter), taste-test the dough and, if necessary, add a little more salt.This recipe does not have to be very exact. I normally don't use measuring cups, I just use a straight-sided cup and guestimate. Most novelty mugs are perfect. For the butter, I just use the markings on the butter packet to guestimate the amount of butter. This recipe is very forgiving.