To make the cumquat chili salt, pound the chili to a paste using a mortar and pestle. Add the salt and lightly pound until well combined. Divide the chilli salt among four small dipping bowls and top with two cumquat halves.
Place the beer, Thai basil stalks, ginger, chili and lemongrass in a saucepan large enough to comfortably hold the mussels. Cover with a lid and bring to the boil for 2 to 3 minutes.
Add the mussels, then cover and cook, shaking the pan often, for about 5 minutes or until the shells have opened. Using a slotted spoon, transfer the mussels to a large serving plate and pour the broth into a serving bowl. Scatter the reserved Thai basil leaves over the mussels and broth.
To eat, invite everyone to juice the cumquats into their chili salt. Dip the mussels in the broth, followed by the chili salt, and enjoy with banh mi, if you like.