Instructions

1. 1
   In a 3-quart mixing bowl, beat the sugar into the egg yolks with a wire whisk until the mixture is pale yellow and forms a ribbon.
2. 2
   Beat in the flour.
3. 3
   While beating continuously, pour the hot milk onto the egg yolk mixture in a thin stream.
4. 4
   Pour the mixture into a saucepan and set over moderate heat, stirring constantly.
5. 5
   Bring to the boil slowly while stirring well to avoid lumps.
6. 6
   When it reaches the boil, stir for 2 to 3 minutes to cook the flour well, being careful not to burn it in the bottom of the pan.
7. 7
   Remove from the heat, then beat in the butter followed by the vanilla extract.
8. 8
   Cover the surface with a sheet of parchment paper to prevent a skin from forming and allow to cool.