To make the chilli paste, in a mortar and pestle, add one ingredient at a time, and pound until a paste is formed. Begin with the lemongrass, followed by galangal, kaffir lime leaves, garlic, red shallots and dried chilli. Finally, add the fermented shrimp and fish pastes and incorporate well. Transfer to a bowl and set aside. Add a few tablespoons of water to the mortar, cleaning both mortar and pestle with the water. Pour water out and reserve for later.
Slice the banana trunk hearts into 5cm lengths, then cut into quarters. Place them in a water bath with lime juice to stop them discolouring. Set aside.
Heat oil in a wok over high heat. Add the chilli paste, and sauté for 3 minutes or until fragrant. Add the chicken and stir-fry for a further 3 minutes.
Add the reserved water from the mortar, plus enough water to just cover the chicken. Bring to the boil, skimming any impurities that rise to the surface. Add the banana trunk and cook for 5-8 minutes until chicken is cooked and the banana trunk softens.
Add the kaffir lime leaves, fish sauce and glass noodles. Cook for a further minute.
Garnish with spring onion and coriander to serve.