Spread butter onto both sides of bread. Cut a 2-inch circle out of the center of the bread. Heat a skillet over medium-high heat. Place bread in the skillet and toast for 1 minute; flip and toast the other side for 1 minute. Crack the egg into the hole of the bread; cook until egg to desired doneness, 2 to 4 minutes. Place Muenster cheese on top of the bread and egg; cook until cheese is melted, 1 to 2 minutes.