The secret is to use homemade vanilla extract. The sponge recipe really benefits from the quality that is homemade. Shop bought extracts, even a quality one, simply aren't the same. The reason you need homemade is because then you get all the different flavour compounds that contribute towards that lovely vanilla taste. Synthetic vanilla only contains vanillin, the primary chemical responsible for what you taste. However, vanilla pods contains aout 200 chemical compounds and it is tasting the combination of these that makes the flavour of vanilla such a good one to so many people. By using homemade vanilla extract you get a high concentration of all of these compounds which go straight into your baking.