Blend flour, sugar, butter, and eggs in a food processor until smooth.
Add the ginger and blend briefly to combine.
Divide the mixture between four small buttered microwave-safe bowls.
Microwave for eight minutes until the sponges have risen and cooked through.
Turn the sponges out onto plates and set aside.
Gently heat milk, cream, and lemongrass in a saucepan until steaming.
Remove from heat and let the mixture infuse for ten minutes.
Whisk the caster sugar and egg yolks together in a bowl.
Remove lemongrass and gradually whisk the milk into the egg mixture.
Simmer on low heat while stirring until the custard coats a spoon.
Strain the custard into a jug and pour over the sponges.