No coconut milk at all. We were shown how these cakes were made by Mrs Tu in our visit to the Tu family home in Hue, and the mixture for the bánh bèo consisted simply of 10 parts of rice flour to one part of tapioca flour, water, peanut oil and salt. The toppings were cooked shrimps with shallots, and chopped scallions in oil.
I just came across this wonderful recipe for bánh bèo by Australian-Vietnamese TV chef, Luke Nguyen:
https://www.cookingchanneltv.com/recipes/luke-nguyen/steamed-rice-flour-cakes-with-dried-shrimp-banh-beo-1960979
