Start by cooking the rice according to the package instructions. Once cooked, chill it in the refrigerator for at least 1 hour (preferably overnight) so it's cold and firm, perfect for frying.
While the rice cools, soften the butter by leaving it on the counter or microwaving it in short intervals. Mash the butter with a fork and mix in the soy sauce. It's fine if the mixture doesn’t combine completely.
Next, heat a splash of oil in a large skillet or wok over medium heat. Add the whisked egg and scramble it, using a spatula to break it into small chunks. Once cooked, remove the egg from the skillet and set it aside.
Add another splash of oil to the skillet, then add the carrots, peas, and onion. Sauté for about 4 minutes or until the vegetables are tender and warmed through. Season with a pinch of salt and pepper. Remove the veggies from the skillet and set them aside with the egg.
For the fried rice, add another splash of oil to the skillet and then add the cold, cooked rice. Toss it every 30 seconds to warm it through and lightly brown it, cooking for about 4-5 minutes. Adjust the heat as necessary to avoid burning.
Drizzle the butter-soy sauce mixture over the rice, tossing to coat evenly. Cook until the rice is completely coated and there are no more visible white grains of rice.
Return the veggies and scrambled egg to the skillet. If using sesame oil, drizzle it over the rice and stir everything until well combined. Taste and adjust seasoning with more soy sauce if desired. Serve immediately.