Melt the butter in a large saucepan over a medium heat. Add the pancetta, onion and celery, place a lid on top and sweat gently until soft. Add the stock and bring to the boil.
Remove the outer leaves off the corn on the cobs, then cut the kernels off with a sharp knife. The best way to do this is to stand the corn on its end and slice down the sides – but mind your fingers!
Leaving the kernels to the side for a moment, add the cobs and potato to the saucepan and bring the stock back to the boil, then simmer for about 10–15 minutes until the potato is cooked.
Lift out the cobs with a pair of tongs and throw them away. Add the kernels and simmer for another 5 minutes. Taste the soup and season with salt and pepper if necessary, then whiz the soup with a hand-held blender until smooth.
Stir in the cream and bring almost back to boil. Ladle into bowls, garnish with parsley if you like, and serve with some extra crispy pancetta and a piece of chilli cracker.