Step 1

In a small saucepan, Heat milk to boiling point and remove from the heat.

Step 2

In a heatproof mixing bowl, beat egg yolks until smooth. Gradually add the granulated sugar and continue beating until pale yellow. Beat in the flour.

Step 3

Pour the hot milk into the egg yolk mixture in a steady stream, beating constantly. When all the milk has been added, place the bowl over (not in) a pan of boiling water, or pour the mixture into the top of a double boiler. Heat, stirring constantly, until thickened. Cook 2 minutes more, then remove from the heat. Stir in the butter and vanilla. Cover with plastic wrap and allow to cool.