Using an electric mixer, beat the egg yolks and sugar together until pale and creamy. Stir in the flour. Add the vanilla or lemon extract and gradually pour the hot milk into the yolk mixture, stirring to combine. Return mixture to the pan and cook over low heat, stirring until thickened, for about 3-4 minutes. Do not boil. Remove from heat and transfer to a bowl. Cover with a piece of baking paper to prevent a skin from forming. Refrigerate until cooled.