The winner: Cafe Demel.
However, don't get your hopes up. Cafe Demel edged out Cafe Sacher because the mini Sacher Tortes were aesthetically more pleasing and the cake was a touch less dry. Even after trying it at both locations that claimed to have invented it though, I'm sad to say I'm not a fan. And this is coming from a chocolate and dessert maniac. I was rooting for it with all my might, but somehow, a chocolate cake with jam filling covered in chocolate just didn't work. However, it's still a must try. Maybe you'll have a better experience than me!