A classic filling for fresh fruit tarts, èclairs, and cream pies, pastry cream is really just a flour-thickened custard sauce, which also means that it can take on a near-endless variety of flavorings, from spices and citrus to chocolates and liqueurs.

Fill the large bowl halfway with ice and water and set it aside. Pour the milk into the medium saucepan. Use the tip of a paring knife to cut the vanilla bean in half lengthwise. Turn the knife over and use the dull side to scrape the seeds into the saucepan, then add the pod. Heat until the mixture just begins to simmer. Remove from the heat and let steep for 30 minutes. (If using vanilla extract instead of a vanilla bean, skip this step and add the extract later.)

Heat the milk to just below the boiling point and remove from the heat. In a medium bowl, whisk together the egg, egg yolks, and sugar until well blended and smooth. Add the flour and whisk vigorously until the mixture is very smooth. Pour about ½ cup of the hot milk into the yolk mixture, whisking constantly to temper the yolks. Slowly pour the yolk mixture back into the hot milk, whisking all the while.

Heat the mixture, whisking constantly to prevent the flour from lumping, until it reaches a boil. Continue to cook and whisk for another minute, until the pastry cream is very thick. Remove from the heat and whisk in the butter (and vanilla extract, if using). Strain the pastry cream through the strainer set over a medium bowl to remove any lumps or tiny bits of egg. (Save the vanilla bean: Rinse it thoroughly, allow to dry, then use it to make vanilla sugar).

Press a piece of plastic wrap directly on the surface of the pastry cream, then set the bowl into the bowl of ice water. Once the pastry cream has completely cooled, use or store in the refrigerator until needed.