Preheat the oven to 180°C/Gas 4 and line a baking tray with a silicone baking sheet or greaseproof paper. In a large bowl, cream the butter and sugars, then beat in the eggs. Add the flour, spoonful by spoonful. When all mixed in, add the vanilla and chocolate chips or pieces. Spoon the mixture on the prepared baking tray in 2 parallel oblongs, spaced well apart to allow for spreading. You may need more than one baking tray.
Bake for 35–40 minutes until just brown on the edges. Remove from oven and let cool a little (leaving oven on). Cut the oblongs across into 1.5 cm thick biscotti and return to the oven. Switch off and leave the biscotti in until they’re cold, turning after 20 minutes.