In a 10 ½-inch (27 cm) crepe pan over medium heat, melt one-quarter of the butter. Pour in one-quarter (½ cup/125 ml) of the egg mixture. Cook until the surface of the omelette is still slightly runny. Place 1 slice of cheese at the centre of the omelette. Fold two opposite sides of the omelette over the cheese, then fold over the other two sides to form a square. Set aside on a plate. Repeat with the remaining butter, egg mixture and cheese.