Mix the egg yolks, sugar, flour and half the milk until smooth .
With the machine running, add the remaining milk through the feeder tube until blended.
Put into a saucepan and cook gently over low heat, stirring well until thick.
Remove from the heat, add the butter and vanilla extract, and cool.
Use for filling cakes, cream puffs or the base of fruit tarts.
Variations For a lighter filling, mix the cold custard with an equal quantity of softly whipped heavy cream.
The custard may be flavored with a liqueur or a little coffee extract.