Heat the oven to 180C/160C fan/gas 4, and line a baking tray with baking parchment. Put the flour, sugars, baking powder and a large pinch of salt in a large bowl and stir together.
In a separate bowl, whisk together the oil, egg, vanilla and 3 tbsp hot water. Gradually mix the wet ingredients into the dry until the mixture comes together into a dough. Gently knead the dough until smooth – it will be quite dry, so add another 1 tbsp water if it’s too difficult to knead after a few minutes. Divide the dough into two equal pieces, shaping each into a 25 x 8cm log. Put the logs on the prepared baking tray and bake for 25-30 mins. Remove from the oven and leave to cool on the tray for around 15 mins.
Using a sharp knife, cut the slightly cooled logs into 1-2cm-thick slices crosswise. Return the biscotti to the lined baking tray and bake for 15-20 mins more (the oven should still be at 180C/160C fan/gas 4), turning the tray around halfway through. Leave to cool slightly on the tray, then transfer the biscotti to a wire rack and leave to cool completely. If you like, dip one end of each biscotti into some melted chocolate, then leave to set on a baking tray lined with baking parchment. Will keep in an airtight container for up to five days.