Preheat your oven to 180C/160 Fan/350F/Gas 4. Lightly grease three large baking sheets.
Cream the butter and both sugars together with a mixer until smooth, then beat in the egg. Add the oats, flour, bicarbonate of soda, baking powder, vanilla essence and a small pinch of salt and mix until thoroughly combined. Stir in the peanut butter and chocolate chips, or your choice of chocolate, nuts, raisins etc, making sure that that they are evenly spread through your mixture.
Take a large walnut-sized spoonful of the mixture and roll into a ball and place on your baking sheet. Continue until you have used all the mixture, placing the biscuits well apart on the tray. The mixture should make 18-20 biscuits.
Bake for around 15 minutes, by which time the biscuits should be a lovely golden brown colour. Remove from oven and leave on the tray for 2-3 minutes, then remove to a wire rack until cooled.
Best enjoyed within two days – if they last that long!