Ditalini soup with borlotti beans and cavolo nero

Ingredients

• 150 gm dried borlotti beans, soaked in cold water overnight
• 50 gm butter
• 1 leek, washed and thinly sliced
• 80 gm pancetta, coarsely chopped
• 2 cloves garlic, finely chopped
• 1½ litres chicken stock (6 cups)
• 100 gm dried ditalini (see note)
• 100 gm cavolo nero, finely shredded
• To serve: extra-virgin olive oil
• To serve: shaved parmesan