1. Combine first 5 ingredients in a shallow dish.
2. Spread each of 3 bread slices with 1 1/2 tablespoons hazelnut-chocolate spread; arrange 1/3 cup banana slices over each bread slice. Top sandwiches with remaining 3 bread slices.
3. Heat oil in a large nonstick skillet over medium-high heat. Working with 1 sandwich at a time, place into milk mixture, turning gently to coat both sides. Carefully place coated sandwiches into pan. Cook 2 minutes on each side or until lightly browned. Cut each sandwich into 4 triangles. Sprinkle evenly with powdered sugar.