Preheat the oven to 180 C or 350ºF and grease your baking tin or a skillet (size about 9 inch in diameter) with vegetable oil.
In a large mixing bowl, mix together the plain flour, baking powder and a pinch of salt.
In another bowl, add the softened butter, white and brown sugar and peanut butter and using a handheld whisk, beat the mixture until nice and smooth.
Add the egg and vanilla and mix on high speed to combine. Don't worry if the mixture looks a bit curdled at this stage.
Using a stiring wooden spoon or your handheld whisk on the lowest speed, add the dry flour ingredients to the wet ingredients and mix until completely combined.
Gently fold in the chocolate chips and distribute them evenly.
Scoop the cookie mixture into your skillet or a round tin and bake for 25-30 minutes, until golden brown.
Serve hot (or cold) with homemade vanilla ice cream or chocolate sauce.