Preheat the oven to 180°C/160°C fan/350°F/gas mark 4. Line a baking sheet with baking parchment.
Whisk together the treacle, sugar, oil, milk, vanilla and lemon zest in a large bowl until well combined.
Whisk in the spices, baking powder and salt, then add the flour and stir with a spatula to form a stiff dough. It should not be sticky at all. If it feels sticky, mix in a spoonful of flour, or if it is too dry and won't come together, add a teaspoon more milk.
Add the chopped crystallised ginger and mix through until evenly distributed. You may find it easiest to use your hands.
Divide the dough into two even pieces and 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 40-45 minutes until they are 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 logs have cooled, use a sharp serrated knife to cut them into diagonal slices about 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. If you want really crunchy biscotti you can leave them in the oven for a further 5 minutes.
Remove from the oven and transfer to a wire rack to cool. Once they have cooled completely, store in an airtight container.