Melt butter in microwave or in a small pot.
Slice bread into 8 pieces to make 4 sandwiches. Spread each piece of bread with aioli. In each sandwich, stack pulled corned beef, sauerkraut and cheese before adding the top piece of bread.
Heat a large nonstick fry pan on a low-medium heat. Use a pastry brush to brush the pan with melted butter. Place a sandwich into the pan and brush the top piece of bread with butter too. Cover frypan with lid and cook for about 3 minutes on each side, until bread is golden and cheese is melted and oozing.
Alternatively, cook toasties in sandwich press or toasted sandwich machine. Repeat with remaining sandwiches and enjoy!