Vanilla extract
Vanilla beans steeped in alcohol, a liquid with a strong vanilla flavour.
Vanilla extract is sold with the baking supplies in most supermarkets, otherwise in delicatessens or online. Choose pure extract rather than artificial vanilla flavouring.
Substitute:
The scraped seeds of one vanilla bean replace about a teaspoon of extract and taste even more delicate. Vanilla sugar also works, but it adds sugar, so reduce the sweetness in the recipe slightly.