Heat the oven to 180°C. Cover a baking sheet with wax paper.
Put the butter into the bowl of an electric mixer. Whisk the butter together with the caster sugar and the dark brown sugar until pale in colour and airy. Beat in the egg until the mixture is homogeneous. Stop the electric mixer then pour in the peanut butter. Stir well.
Sieve the flour and baking powder together. Gently add to the mixture using a spoon or the electric mixer set on the lowest speed. Add the chocolate chips. The dough should be firm yet easy to handle now.
Using your hands, shape the mixture into balls then place them on the baking sheet. Be careful not to put them too close to one another as they will spread out while baking.
Put in the oven for 15 to 20 minutes, until they take a nice golden colour. Note that the center should still be a bit doughy.
Leave the cookies to cool down on the baking sheet then serve.