I am trying to cook Buta no Kakuni or Japanese pork belly but it's tough I have tried cooking "Butta no kakuni" or slow braised pork belly. I watched a video on making kakuni from a check in Nagasaki. In it he used a Le Creuset dutch oven to cook the pork belly.


Cut the pork belly into 4 inch strips, cut those in to 4"x4"
lightly brown the cubes
add a large scallion, star anise, and a stick of cinnamon
cover with water add salt
after cooking - remove the water
add mirin, sake, soy, sugar and cook on low until well reduced.


It was cooked in a dutch oven - brought to a boil in the first phase, then simmered for 1 hour. The second phase was simmered for 3 hours. It's soft, but not super soft like I had in Japan, where it is like eating butter with a very luxurious and buttery caramelized consistency. I have to admit, I did not use a thermometer to check the meat's temperature through various stages

My question is: how does one keep the pork from becoming tough and how does one cook meat in general in a way that it stays soft, allows flavor to thoroughly permeate but stays whole without flaking and breaking too easily when plating.

I am not a pro, but I am passionate about this!
In his book, Masaharu Morimoto says to cook it for 8 hours for the initial phase (in a 240F oven). He also includes brown rice in the initial cooking, which he says helps to tenderize the meat. He then refrigerates it over night and then cooks it for another 2 hours the next day.  The recipe is reproduced on the Chubby Hubby blog.

If that doesn't get the texture you want, you could try a slow cooker or sous-vide. (Sous vide should be able to nail the texture if you get the temperature/time.)