Preheat the oven to 180°C/160°C fan/350°F/gas mark 4. Line a baking sheet with baking parchment.
Whisk together the oil, milk, sugar, espresso powder and vanilla extract in a large bowl.
74 g (⅓ cup) olive oil (or another kind of oil such as sunflower/vegetable), 90 g (6 tablespoons) hazelnut milk (or another plant milk), 150 g (¾ cup) caster or granulated sugar, 3 Tablespoons instant espresso powder, 1 teaspoon vanilla extract
Whisk in the baking powder and salt then stir in the flour to form a softish sticky dough. If it is wet rather than sticky you can add up to an extra 25g of flour.
1 ½ teaspoons baking powder, ½ teaspoon salt, 300 g (2 ½ cups) plain (all-purpose) flour
Add the hazelnuts and stir until they are well distributed throughout the dough.
100 g (¾ cup) hazelnuts
Divide the dough into two even pieces and using floured hands shape each one into a log about 6cm wide, 18cm long.
Place them well spaced apart on the lined baking sheet and use your hands to slightly flatten them and square off the edges for perfectly shaped biscotti.
Bake the logs for about 35 minutes until they are just firm to the touch.
Remove the logs from the oven and set them aside on a wire rack to cool for 20 minutes. Turn the oven down to 160°C/140°C fan/325°F/gas mark 3.
Once the 20 minutes is up, use a very sharp serrated knife to cut the logs into diagonal slices about 1 ½ cm wide. Don't put too much pressure on the knife, try and gently saw through. Handle the sliced biscotti gently as they are delicate and prone to breaking.
Lay the slices cut side down on the baking sheet and return them to the oven for 10 minutes. Turn the slices over and bake for another 10 minutes. They should look dry though they will firm up more as they cool. If you want really crunchy biscotti you can leave them in the oven for a further 5-10 minutes.
Remove from the oven and transfer to a wire rack to cool.
Place ⅔ of the chopped chocolate in a heat-proof bowl over a pan of gently simmering water, do not let the base of the bowl touch the water.
200 g (7 oz) dark chocolate or vegan milk chocolate
Heat, stirring often, until the chocolate is just melted, do not let it get too hot. Remove the bowl from the heat and add the remaining ⅓ of the chocolate a bit at a time, stirring well after each addition until it is melted before adding more.
Pour the chocolate into a deep, narrow container, such as a mug. Dip each of the cooled biscotti in the chocolate, shake off the excess, then place them on a baking parchment lined tray to set.