For the prawns, remove their heads, slit down their backs and de-vein.

Crush a few pieces of lemongrass and place in about 2 cm of water in the base of a double boiler and bring to a boil. Put the prawns in a colander and place them over the boiling water and lemongrass. Steam the prawns covered with a lid until they are cooked (about 6 minutes).

For the noodles, bring a large pot of hot water to the boil and toss in the noodles. Boil for a few minutes until they are cooked. Drain the noodles in a colander and refresh in some cold water.

For the soy beurre blanc, mix together all the ingredients except the butter and bring to a boil. Whisk the frozen butter cubes a few at a time into the sauce. Wait until the cubes have been completely incorporated before adding some more butter. It is important to keep on stirring continuously.

Serve immediately or keep this sauce warm in a thermos flask until required as reheating will cause the sauce to split.