Combine sour cream, cheddar cheese, salt and pepper; set aside.

Cook potatoes in salted boiling water until tender, about 10-15 minutes. Drain; cool slightly. Flatten the potatoes to about 3/8-inch thickness.

For each sandwich: Butter one side of two slices of bread. Spread a 1/4 cup of cheese mixture on both unbuttered sides of bread. Sandwich 3-4 potatoes, about 1 ounce of bacon and a generous tablespoon of scallions between the cream cheese sides of the bread.

Grill sandwich on hot buttered grill or sandwich press until filling is heated through and melty. Cut in half and serve warm.