Halve vanilla bean lengthwise and scrape out seeds with a sharp knife. Heat milk in a pot, add vanilla seeds and pod, stir and bring to a boil and remove from heat. Mix egg yolks with sugar in a bowl until creamy and then add flour. Gradually add warm milk to egg yolk mixture while stirring. Pour all back into the pot. Remove vanilla pod and beat over low heat until cream thickens, but do not cook! Pour custard with grapes into a pudding mold and cool completely for several hours and leave to set.
Then rush to a plate and serve garnished with grape leaves.