This is a very simple cake from the Mekong Delta. Not just popular in southern Vietnam but also in Cambodia. They're named for their resemblance to a swallow's nest (a.k.a. tổ yến). A traditional recipe would likely call for cơm rượu for leavening. As with bánh bò, a well made bánh tai yến should have rễ tre (the "honeycomb" texture").
