This a tea house run by the Taiwanese Buddha Light Temple.
The food served is Chinese food, strict Chinese Buddhist style (no eggs, onion, garlic, leek, chives). Dairy products are permissible in Chinese Buddhist vegetarian food but I didn't encounter any when I ate there.
Very conveniently located next to the Dongguk station. Once you come up from exit 2, you will see a sign of the cafe pointing into an alley. The building looks closed but you go through the door and head towards the basement.
They have started serving lunch buffet 11:30am to 1pm for 6000W. It's all you can eat, with rice and several dishes, soup and fruits. I was there twice and the soup was really fantastic delicious. The dishes were all Chinese food, though daikon kimchi was available. What I like is that they are like home cooked food.
You can also order from the menu if you don't want to eat the buffet. There are noodles and rice dishes.
I got greedy and ordered a plate of pot stickers (5000W) on top of the buffet. They were pretty good.
The all-you-can-eat lunch buffet for 6000W was fantastic value for money.
They spoke very limited English. I spoke Mandarin with them.
Unfortunately, they close rather early in the evening, at 7pm
Cons: Close rather early in the evening