1. Put all ingredients, except corn flour, in a saucepan with 125mls of water and bring to the boil.
2. Mix the corn flour with 2 tablespoons of cold water until smooth, then add to the pan.
3. Bring the sauce back to the boil.
4. Remove Kaffir leaves.
5. Keep for up to 4 weeks in an airtight container in the fridge.