Bring the milk, sugar and vanilla to boiling point.
Melt the butter in a large heavy-bottomed saucepan. Sift in the flour little by little, stirring well. When the mixture is bubbling, add the boiling milk in a steady stream; bring back to the boil and beat all the time until thick and smooth. Remove from the heat and allow to cool a little. Beat in the egg yolks and the kirsch.