Heat sauerkraut in a saucepan or in the microwave, then drain. Butter one side of each piece of bread. Lay 6 slices of bread on a preheated griddle, butter side down. Mix equal parts Dijon mustard and mayo together, then spread mixture on each of the 6 slices of bread, then layer corned beef, cheese and sauerkraut on each piece. Top each sandwich with another piece of buttered bread, buttered side up. When golden brown, flip and grill other side.