After the beef has marinated, dust ½ cup of potato starch (or corn starch) on the beef and use your fingers to loosely coat the beef with it. Let it sit for 10 minutes
While you are waiting for the meat to marinate, heat up a heavy bottom pot (or wok) and set the stove to medium heat. Add at least 1 inch of oil to the pot and wait for it to get hot. (It should reach a temperature of 375F). I like to poke a wooden chopstick, or the handle of a wooden spoon into the oil and wait for it to bubble. Once it bubbles, the oil is hot enough.
Once the beef has marinated for 10 minutes, dust the other ½ cup of potato starch (or corn starch) on the beef again to lightly re-coat the beef and lightly dust off any excess starch. Deep fry it immediately.
Using your fingers separate the beef if they are clumped together and deep fry it in the oil in small batches (make sure the oil is hot but not smoking - to test if the oil is hot enough, add 1 small piece of beef first and see if it instantly sizzles)
Deep fry the beef for 1-2 minutes, and remove it from the pot onto a plate with parchment paper or newspaper.
Once all the beef has been cooked once (first fry), add all of it back in for the second deep fry for 1 more minute.