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").  
   
Like all traditional Asian pastries, the ingredients are simple to track down.  And...guess what?  They're gluten free!

Ingredients:
-250g rice flour
-15g tapioca starch
-150g sugar
-175g water
-100g coconut milk      
-baking soda...just a tad
-enough oil for to deep fry with

Method:
Mix everything together to make a smooth batter.  What's that? You see lumps in the batter?  Pass the batter through a strainer.  

Fill a deep pot about 1/2 full with oil...or...fill the pot with oil until it's 1/2 empty. ;)  Either way, the pot needs to be able to hold twice as much oil as you're filling it with. lol  Heat the oil over medium high heat until it reaches 350'F.  

Using a small ladle, give the batter a quick stir, then pour a ladle's worth of batter (+/- depending your preference) straight into the oil.  By "straight into the oil" I mean, pour the batter in a steady stream, do not move the ladle/your arm while pouring.  In about 10-15 seconds the cake should float to the surface, continue frying until they are light golden.

Note:
Only a pinch of baking soda is needed, seriously! 

Baking soda reacts with heat (releases carbon dioxide), while the starches in the flour expands and coagulates to capture the gases; this gives the cake their trademark honeycomb texture.  Too much baking soda = too much "reaction".  In other words, too much gas is released too fast for the starches to hold them together.  The result?  The batter will break into tiny bits and pieces.