For the sponge, place the flour, sugar, butter and eggs into a food processor and blend until smooth. Add the ginger and briefly blend again.
Divide the mixture between four small microwave-safe pudding bowls greased with butter.
Place into the microwave for eight minutes, or until sponges have risen and are cooked through.
Turn out the sponges onto four plates and set aside.
For the custard, place the milk, cream and lemongrass into a saucepan over a low heat and gently heat until steaming, but not boiling. Remove from the heat and leave to infuse for ten minutes.
Place the caster sugar and egg yolks into a bowl and whisk together.
Remove the lemongrass from the milk mixture and gradually pour the milk mixture into the bowl of eggs and sugar, whisking constantly.
Return the mixture to a low heat and simmer, stirring continuously, until the custard coats the back of a spoon, then strain into a jug.
To serve, pour equal portions of lemongrass custard over each sponge pudding and serve.