De-seed the chilli and finely chop. Peel and thinly slice the garlic.
Heat one Tbsp of oil in a wok or heavy based pan, then stir-fry the chilli and garlic on medium heat.
When the garlic is starting to colour, add the butter (if used) and the prawns. Stir-fry for a minute. Add the lemon juice, let it bubble for a few seconds. The prawns should be changing colour by now.
Turn off the heat and put a lid on the pan. Rest for a couple of minutes for the prawns to cook through properly in the steam (this stage is not needed if you're using pre-cooked prawns). Add the lemon zest and herbs of your choice.
Serve warm, at room temperature or cold.