Slice vanilla beans lengthwise and scrape out the seeds
In a saucepan, mix water, corn syrup, and vanilla seeds
Add the vanilla bean pods and bring to a gentle simmer over low heat
Stir occasionally and let the mixture reduce slightly for about 10 minutes
If using cornstarch, mix with a tablespoon of water and stir into the mixture to thicken
Remove from heat, stir in vanilla extract, and let cool
Strain out the vanilla bean pods (optional) and transfer to an airtight jar
Store in the refrigerator for up to 6 months