You don't drink it from a cup — you sip it through a bamboo straw straight out of a wooden vessel, topping it up with hot water as you go.
Tongba is fermented finger millet served in a tall wooden pot, topped with hot water and sipped slowly through a filtered bamboo straw, with refills added as the flavor fades. It's especially associated with the Limbu people of eastern Nepal and is a common warm-up drink in Himalayan teahouses.