Yield: About 2 cups/500 ml pastry cream, enough to fill one batch of choux pastry.
Put the milk in a saucepan. Scrape the vanilla seeds into the milk and throw in the pod. Bring just to a boil, remove from the heat, cover, and set aside to infuse for 10 to 15 minutes.
Beat the yolks and sugar to pale thick ribbons. Gradually beat in the flour. Whisk the milk into the egg mixture in a thin stream. Add the flavouring and bring to a boil over medium heat, stirring constantly until the mixture thickens. Strain and set aside to cool with a buttered piece of waxed paper on top to prevent a skin from forming. When cool, fold in the whipped cream and transfer to a piping bag with a small tip (I find that one with a jagged edge works best). To fill the puffs, poke the tip into the bottom or side of each puff ball and pipe in pastry cream. You may need to do one or two before you get a feel of what amount is right for pumping in.