1. Spread cut sides of pretzel rolls evenly with mustard. Layer 4 roll halves each with 1 cheese slice, 2 ounces pastrami, and a second cheese slice. Top with remaining 4 roll halves.
2. Spread outside of sandwiches with softened butter. Cook in a nonstick skillet over medium heat, in batches, 2 minutes on each side or until cheese melts and bread is golden brown.