1. Brush both sides of each slice of bread with the butter and spread the mango puree on both sides of each slice of bread. Arrange the watercress over the top of one side of a slice of bread. Place the cheese on top of the watercress and top with the second slice of bread.
2. Heat a griddle on top of the stove over medium-high heat. Place the sandwich on the griddle and grill for about 1 minute on each side, or until the bread is golden brown and the cheese is melted. Slice the sandwich into 8 small serving pieces.