Unlike over easy eggs, sunny-side up eggs are only cooked on one side. The yolk (the "sunny side," if you will) never touches the pan.
Heat a 10-inch nonstick skillet over low for 3 minutes. Crack the eggs into a bowl. Add the butter to the hot pan and swirl until melted. When the butter stops foaming, slowly pour the eggs into the skillet. If necessary, gently nudge the yolks with a wooden spoon to space them evenly in the pan.
Working quickly, season the eggs with salt and pepper, then cover the skillet and cook until the whites are completely set and the yolks are bright yellow, about 3 minutes. Slide out of the pan and onto plates.