Bring a large pot of salted water to a boil, and add the yuca chunks. Boil over high heat until the yuca is very tender, for about 30 minutes, with the lid on.
Check occasionally to make sure the water hasn't dried out. If it has, add enough water to cover the yuca and continue boiling
Once done, add the yuca chunks to a bowl (don't throw away the water yet!) Remove the fibrous stem from the center of the roots.
Add 1/4 cup of the starchy water you cooked the yuca in, along with the butter to the same bowl, and get to mashing! You can use a pestle, potato masher, or whatever tool you like for mashing. If you want the yuca mash creamier, add more water. It'll take a few minutes, and a good arm workout!
Mash in salt, to taste and enjoy as a side with sautéed red onions!
We sautéd our red onions with olive oil and red wine vinegar