Heat medium skillet over medium heat.
Butter bread on one side of each slice.
Place one slice of bread, buttered side down, in the heated skillet.
Layer corned beef evenly over the bread slice.
Add sauerkraut on top of the corned beef.
Place mozzarella cheese slice over the sauerkraut.
Top with remaining slice of bread, buttered side up.
Cook for 3-4 minutes until bottom is golden brown.
Carefully flip sandwich and cook another 2-3 minutes until second side is browned and cheese is melted.
Remove from heat and serve immediately.