What @chromedome said is correct, in this case, you cannot sub cassava flour and tapioca. Think of it like cornmeal and cornstarch. 
These things were life changing for me, but many people I know are like "meh". So, YMMV, and sorry this is so long but I am passionate about these (one of my favorites!). What I do, is actually kind of similar to @Kim Shook's recipe in her original post above. I mostly use this recipe, converted to cups and a lot more cheeeese:
https://www.youtube.com/watch?v=4H3-wryqcpk
- 200g sour starch or tapioca starch
- 1 cup milk
- 2T fat (butter, oil, duck, whatever)
- 1 egg (if this is too dry, try with 2 eggs, I prefer 1 egg, going back through my notes)
- 1/2-1c hard cheese
- optional salt -- I like these salty, I think I add 1-2t salt
Bring milk and fat to a boil, immediately pour over starch and hit it with the hand mixer + beater attachments. It will clump up and turn into a "fluff"**, and don't fret if it runs up the beaters. If it's a runny mess, then you didn't get the milk hot enough. It'll still turn out okay, but texture will end up being more like mochi.
Let this cool, then beat in the eggs, and once incorporated, mix in the cheese. I shape these into small balls using wet hands (keep the tap running to help), then freeze. Try baking different temps ranging 350-425F, I always forget
My way TMI comments for anyone that may find this helpful, I used to make these monthly during my wheat-free phase and am just excited to be able to contribute something to this place where I've learned so much:
Many recipes will tell you you can mix this by hand. Anyone throwing off fleeting romances off this, stop right there, it's way harder than trying to whip your own whip cream by hand.
Don't dilly-dally with the milk->starch->beater. You have to get the starch to gelatinize with the hot milk to get the "authentic" texture of these, that's more bread like.
OKay to be heavy handed with the cheese, or lighter, all good
Cheese is historically and authentically green kraft parmesan, but I've used all sorts, grated, shredded, parmesan, pecorino, asiago, mix them up, use irregular sizes, etc.
There are dozens of variations, some people will make more of a runny batter, or add flour. Others don't heat the milk with the starch, etc. This will make more of what you find at the brazilian BBQ restaurants. They are all delicious and tasty. If you don't boil the milk, your buns end up more like chewy mochi rice cakes, or gummy tapoica balls in those drinks. That's fine, some like it, but I also enjoy a more fluffy-bread like cheese flavor bomb.
People who make more of a batter use the cupcake tins. I see @Kim Shook's recipe does, too, and that's okay I just like having these for later so I make balls and freeze them. They are amazing with espresso, and dressed with a little butter and honey.
You can sub buttermilk for milk at 1:1, and I have used a bit of yogurt as well in the past, but I forget how much. These things are pretty forgiving when it comes to proportions, as long as you have the liquid boiling.
Finally, if you get sour starch, there are two brands predominantly in the US: Amafil and Yoki. I get better results with Yoki. Apparently, they can be bought in the hispanic markets, and I live/work in heavy hispanic DMA of so-cal and have yet to see them in any stores, only Amazon purchases for me. Be careful, this stuff STINKS REALLY BAD. But don't worry, this umami-filled vomit-poo aroma dissipates into an amazing cheese ball. Actually as gross as the smell is, you kind of want to keep sniffing it. Very strange!
Hope this was helpful, enjoy Again, apologies this was so long. I may have mentioned I really really really like these! 🤪
