Melt butter in microwave, watching carefully, and spread on one side of each slice of bread. Preheat a heavy cast iron pan over medium heat. Place one slice of bread, buttered side down, in pan, and add brie slices. Spread Nutella on other slice of bread and top the sandwich. Cook uncovered for 3-4 minutes, then flip carefully. Continue cooking until bread is crisp and golden on both sides.
