Preheat the oven to 180°C/160°C fan/350°F/gas mark 4. Line a baking sheet with baking parchment.
In a large bowl, whisk together the sugar, soy milk, oil and vanilla extract until well combined.
In a separate bowl, sift together the flour, cacao or cocoa powder, baking powder and salt.
Add the dry ingredients to the wet and stir until it forms a rough dough then add the pistachios and chocolate chips and mix until they are well distributed. Use your hands to bring it together into a ball.
Divide the dough into two even pieces. On a lightly floured surface, shape each one into a log about 5cm (2in) wide. Place them well spaced apart on the baking sheet and use your hands to slightly flatten them and square off the edges for perfectly shaped biscotti.
Bake the logs for 35 minutes until they are firm to the touch.
Remove the logs from the oven and set aside to cool on a wire rack for 20 minutes. Turn the oven down to 160°C/140°C fan/325°F/gas mark 3.
Once the logs have cooled, use a very sharp serrated knife to cut them into diagonal slices about 1 to 1 ½ cm wide. Don't put too much pressure on the knife, try and gently saw through.
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 mostly dry though they will firm up more as they cool. .
Remove from the oven and transfer to a wire rack to cool. If you want really crisp biscotti you can leave them in the oven for a further 5 minutes
Once cool store in an airtight container.