Heat the oven to 180C/160C fan/gas 4, and line a baking tray with baking parchment. Set aside. Put the flour, sugars, baking powder and a good pinch of salt into a large bowl, then stir to combine.
Whisk the oil, egg, vanilla extract and coffee together in a separate small bowl. Slowly pour the wet ingredients into the dry, mixing until you have a dry dough. Fold in 100g of the chocolate chips. Knead the dough until smooth – it will be quite dry, but if it’s too difficult to knead, add another 1 tbsp water. Divide the dough in half, shaping each piece into a 25 x 8cm log, and transfer both to the prepared tray. Bake for 25-30 mins, then leave to cool on the tray for 15 mins.
Reduce the oven to 160C/140C fan/gas 3. Slice the biscotti logs into 1-2cm-thick slices crosswise using a sharp knife, you should have about 16. Return to the tray and bake for 15-20 mins more, turning the tray around halfway through. Leave to cool slightly on the tray, then transfer to a wire rack and leave to cool completely. Melt the remaining chocolate chips in the microwave in 20-second bursts until smooth, or in a heatproof bowl set over a pan of barely simmering water, then dip one end of the cooled biscotti into the melted chocolate. Transfer to a sheet of baking parchment and leave to set. Will keep in an airtight container for up to five days.