Begin by cutting the chicken into uniform 1-inch cubes, ensuring they are roughly the same size for even cooking. Place the cubes in a bowl and sprinkle with salt, pepper, and garlic powder, then toss to coat evenly. Let sit while you prepare the pan.
Heat a large cast-iron skillet over medium-high heat until shimmering. Add the oil and swirl to coat the bottom of the pan. The oil should sizzle lightly when the chicken is added.
Add the coated chicken cubes to the hot skillet in a single layer, making sure not to overcrowd the pan. Let them sear undisturbed for about 2-3 minutes until the edges turn golden and crispy.
Using tongs or a spatula, flip the chicken cubes to sear the other sides. Cook for another 2-3 minutes, until the cubes are golden brown all over and cooked through. The chicken should feel firm but juicy when pressed.
Check the doneness by piercing a cube—juices should run clear, and the interior should be opaque. If using a thermometer, internal temperature should reach 75°C (165°F). Remove the chicken from the pan and transfer to a plate.
If desired, squeeze fresh lemon juice over the hot chicken cubes to brighten the flavor. Serve immediately or incorporate into salads, wraps, or stir-fries for a quick meal.