Heat milk (and vanilla bean) until just boiling, then remove from heat. In medium bowl, beat egg yolks and sugar until pale and thick, then incorporate the flour. Remove vanilla bean. While gently beating the eggs, add hot milk 1/2 cups at a time until eggs have warmed, then transfer everything back into the pan. Whisking continuously, heat on medium-low until boils for one minute. The cream should thicken considerably. (Mix in vanilla extract and/or chocolate or coffee.) Pour into clean bowl to cool.