2. Heat butter in a frying pan over medium heat. Crack the egg into the pan and cook until the white is set but the yolk is still runny, about 3-4 minutes.
3. While the egg is cooking, mash the avocado in a bowl and season with salt and pepper.
4. Spread the mashed avocado on one slice of toasted bread. Place the cooked egg on top of the avocado. Top with the other slice of bread.