For the skewers
For the dipping sauce
Preheat a griddle pan and place over a high heat.
Using a small sharp knife, split 3 of the lemongrass stalks in half, lengthways. Set aside.
Finely chop the white part of the remaining lemongrass stalks and place in a small food processor with the lime leaves, galangal, garlic, ginger, fish sauce, soya sauce and coriander. Process to form a paste. Place the paste in two bowls.
Place the chicken in one bowl and the prawns in another, mix well to combine.
Using wet hands, divide the chicken and prawn mixtures each into 12 portions and shape each around the end of a lemongrass skewer. Allow the mix to rest in the refrigerator for at least 1 hour.
When ready to cook, roll the skewers in the sesame seeds, lightly brush with oil and cook, in batches, for 8 minutes, turning, or until charred and cooked through.
To make the dipping sauce, place the all the ingredients in a bowl and stir to dissolve the sugar.
To serve, place the vermicelli noodles in the lettuce cups and top with the skewers and garnish with the coriander, mint and lime wedges.
Serve with the sauce.