This pastry cream will keep in the refrigerator for 2 to 3 days. If you make it in advance, be sure to chill it quickly and thoroughly.
Place milk and vanilla bean and seeds in a small saucepan. Bring to a boil over medium-low heat.
In a medium bowl, whisk egg yolk and sugar together until pale and creamy. Add flour, and mix to a smooth paste. Add half of the boiling milk, and mix well. Return mixture to pan with remaining milk, stirring to mix well. Boil for 1 minute until thickened, stirring constantly. Pass through a fine sieve.