Use day-old cold rice — this is the most important rule. Freshly cooked rice has too much steam and will clump and turn mushy. Spread freshly cooked rice on a sheet pan and refrigerate for at least 1 hour if you're in a hurry.
Beat eggs with a pinch of salt.
Heat a wok or large skillet over the highest heat your stove has. Add sesame oil.
Add cold rice and press flat. Let sit 2 minutes undisturbed — you want the bottom to get a crust and some grains to toast. Toss, press flat, and repeat once more.
Push rice to the edges. Add a small amount of butter to the center. Pour in beaten eggs and scramble until just set. Immediately fold into the rice before the eggs are fully cooked — residual heat finishes them.
Add soy sauce and toss vigorously over high heat 1 minute. Serve immediately.