I'll be happy to make Sacher Cake to impress you, and maybe as a payment for the washing machine dial *gg*

Potato starch is used to thicken things and to keep them from being too runny, e.g. mixing with the jam to make it stay on top of a tart and not run all over the oven. According to the all knowing internet corn starch and tapioca flour can be used as substitutes.