Take butter and castor
sugar in a mixing bowl and beat with an electric beater till sugar is dissolved. Add brown sugar and keep beating. Add peanut butter and keep beating.
Add egg and beat till
sugar is completely dissolved (some grains of sugar may remain).
Add flour and baking
soda and fold in with a spatula till well mixed. Cover the bowl with cling film and keep it in the refrigerator for 15-20 minutes to set the dough.
Preheat the oven to
170º C. Grease a baking tray.
Divide the dough into
small balls, make a small depression with a fork in the middle of each cookie. Keep them on the greased baking tray.
Keep the tray in the
preheated oven and bake for 20-25 minutes.
Take the tray out of
the oven and set aside to cool. Arrange the cookies on a serving platter and serve.