Blend the caster sugar, cofnflour, plain flour and milk together in a 1.5 litre bowl, mixing well with a whisk. Microwave at 100% (high) for 7 - 7.5 minutes until thick, stirring vigorously with a hand whisk every 2 minutes Lightly beat the yolks in a separate bowl and mix in 6 tablespoons of the hot custard. Blend thoroughly, then stir the yolk mixture into the remaining custard. Microwave for a further 1-2 minutes or until thick. Beat in the butter and vanilla flavouring, then cover with cling film or greaseproof paper. Leave to cool