Unknown. In fact not only do we not know “who”, we don’t even know “when”. Panna cotta almost certainly evolved from custard, which has a long, long history on the continent. However the fact that it’s thickened with gelatin means it can be no older than 200 years, because commercially-produced gelatin wasn’t around before then.
It seems to me, however, that were panna cotta as old as that, we’d have heard about it. Somebody would have mentioned it in some cook book, some travelogue…something. But the fact is that panna cotta doesn’t even appear on culinary radar screens until the middle of the 20th century. That makes a certain amount of sense to me, for reasons we’ll get into later.