Easy Rice and Cheese Souffle

Name: Easy Rice and Cheese Souffle
Ingredients: 1/2 cup butter, plus 1 tablespoon, divided cut into small pieces; 3 cups cooked rice chilled and broken up with hands; 1 2/3 cups milk heated to steaming, 140°F-155°F.; 4 ounces freshly grated Parmesan; 4 ounces shredded Cheddar or Swiss cheese; 1 tablespoon Dijon mustard or 1 generous teaspoon British style prepared mustard; 3 tablespoons minced fresh chives, plus extra for garnish, if desired or 1 1/2 teaspoons dried minced chives; 1 teaspoon coarse kosher salt; 1/2 teaspoon coarsely ground black pepper; 6 eggs separated