Prep Time: 8
Cook Time: None
Serves: Serves: 2
Spread butter over 1 side of each slice bread. Turn 2 slices over; top with 2 slices cheese and the roast beef. Spread mustard over beef; top with pickles if desired. Top with remaining 2 slices cheese. Close sandwiches with remaining bread, buttered sides up.
Cook sandwiches in a preheated panini maker or greased waffle iron until golden brown and cheese is melted, 3 to 4 minutes. (Or sandwiches may be cooked in a large nonstick skillet or grill pan over medium heat, 3 to 4 minutes per side.)