Hi Karen, it is so annoying when panna cottas do that! I know the feeling. I think it has to do with the cream used. if the cream hasnt been homogenized (such as pure cream) then i find it does split. whereas is the cream is thickened cream, then I find it stays together and doesnt form layers. I usually use pure cream in ALL my recipes, but I have recently swapped to thickened cream when making panna cottas for that reason.
If you try it again let me know how you go !
Thanks, alyce