Put the curry paste and peanut butter into a saucepan set over a medium heat, stir together to a smooth paste and sizzle for 1 min. Add the coriander stalks, ginger, lime leaves, coconut milk and stock. Bring to a simmer, then gently boil for 5 mins.
Add the soy, lime juice and fish sauce. Taste and adjust the seasoning with a little more soy if you feel it needs more salt, or more lime if you want it a little more sour. Add the pak choi and prawns, and cook for 1 min until the prawns are pink and the pak choi has wilted. Stir in the rice noodles and cook for 30 secs more. To serve, divide the pak choi, noodles and prawns between 4 large soup bowls. Pour over the soup and top with a handful of coriander leaves, the chilli slices and chopped peanuts.