1. Spread mustard over 1 side of each bread slice. Layer 4 bread slices, mustard sides up, with turkey and cheese slices. Top with remaining bread slices, mustard sides down.
2. Whisk together eggs and milk in a shallow dish. Dip both sides of each sandwich into egg mixture.
3. Melt 1 Tbsp. butter in a large nonstick skillet over medium heat; cook 2 sandwiches in skillet 3 to 4 minutes on each side or until lightly browned. Repeat procedure with remaining butter and sandwiches. Serve immediately with blackberry preserves. Dust with powdered sugar, if desired.