I think it's a bit like flan but without caramel. About custard, sorry i don't know what custard is, Im never try it cause its not popular here. But I just looked it up, even though they look similar I don't think they taste the same. Custard uses egg yolks like egg coffee but with milk, cream, even butter. These will change the taste of custard a lot compared to egg coffee. So I think egg coffee tastes more like flan than custard. Hope this helps
But obviously not with Jaewan. Hope you can still come after 1000 days with your new mate
and i recommend egg coffee if anyone is interested to visit Hanoi