Deseed chilli peppers. Finely slice lemon grass (if using). Chop ginger into chunks. Process the spice paste ingredients (chilli, coriander / cilantro stems and leaves, onion, lemon grass / lemon zest and ginger) with 2 tablespoons oil in a food processor until you have a chunky puree.
Slice chicken into bite sized pieces. Heat a few tablespoons peanut oil in a large frying pan and stir-fry the chicken for a few minutes or until starting to brown. Remove from the pan and keep warm.
Add the spice paste to the pan and cook, stirring for about a minute or until the spice paste smells fragrant.
Add the coconut milk and fish (or soy) sauce and bring to a simmer. Cook gently for about 5 minutes or until the sauce has thickened slightly.
Return the chicken and any juices to the pan and bring back to a simmer. Taste, season and serve hot with basil leaves (if using).