Bring milk to boil and set aside. Mix egg yolks, sugar, and vanilla in a bowl with a wire whisk, until mixture forms a ribbon. Add flour and mix well (will become rather thick). Add half the boiled milk to the yolk mixture, mixing well. Pour yolk mixture into remaining milk, mixing as you go along. Bring to a boil on medium heat, stirring constantly with the whisk. Sauce will thicken as soon as it reaches the boiling point. Reduce heat and cook 2 to 3 minutes. NOTE: Be very careful at this point; sauce will burn very quickly if heat is too high. Pour into a bowl, cover with plastic wrap, and poke a few holes to allow cream to breathe. Refrigerate for 1-2 hours or until completely cooled.