Separate the egg whites and yolks into two clean bowls.
Add the softened butter to the egg yolks and whisk until smooth and creamy.
Add the salt to the egg whites.
Beat the egg whites until stiff peaks form. When you tilt the bowl, the egg whites should stay in place.
Gently fold the egg yolks into the whipped egg whites until just combined.
Heat a non-stick frying pan (about 20 cm / 8 inches in diameter) over medium heat and melt the butter.
Pour the egg mixture into the pan and gently spread it into an even layer.
Cook until the bottom is golden brown while the top remains slightly soft.
Carefully fold the omelette in half using a spatula.
Transfer immediately to a warm plate and serve.