Preheat broiler with oven rack 8 to 9 inches from heat.
Bring 1/4 cup salt and 4 qt. water to a boil in a large covered Dutch oven over high heat.
Meanwhile, microwave milk in a microwave-safe 1-qt. glass measuring cup covered with plastic wrap at HIGH 3 minutes. While milk is heating, melt butter in a 12-inch cast-iron skillet over medium heat. Reduce heat to medium-low; add flour, and cook, whisking constantly, 2 minutes. Gradually whisk in hot milk. Increase heat to medium-high, and bring to a low boil, whisking often.
Add pasta to boiling water, and cook 8 minutes.
Meanwhile, continue to cook sauce, whisking often, 6 minutes. Remove from heat; whisk in cheeses, hot sauce, 1 1/2 tsp. salt, and 1/2 tsp. pepper. Cover.
Stir together panko and olive oil.
Drain pasta, and fold into cheese sauce. Sprinkle with panko mixture.
Broil 1 to 2 minutes or until breadcrumbs are golden brown. Serve immediately.
Southern Pimiento Mac and Cheese: Substitute extra-sharp Cheddar cheese for Monterey Jack cheese. Prepare recipe as directed through Step 5, whisking 2 Tbsp. fresh lemon juice, 1 Tbsp. Worcestershire sauce, and 1/2 cup grated sweet onion into sauce along with cheese. Proceed with recipe as directed, stirring 2 (7-oz.) jars diced pimiento, drained, and 2 (52-oz.) packages fully cooked bacon, diced, into pasta mixture just before topping with panko. Sprinkle with 2 Tbsp. chopped fresh chives before serving.
Barbecue Mac and Cheese: Substitute Gouda cheese for Monterey Jack cheese and crumbled savory cornbread for panko. Prepare recipe as directed through Step 5, stirring 1 lb. pulled pork barbecue (without sauce) into pasta mixture after adding cheese. Sprinkle 1/2 cup chopped green onions over cornbread mixture before broiling, and drizzle with 1/2 cup bottled barbecue sauce after broiling.
Mexican Mac and Cheese: Substitute pepper Jack cheese for Monterey Jack cheese and crushed tortilla chips for panko. Prepare recipe as directed through Step While pasta cooks, sauté 1 lb. Mexican chorizo in 1 Tbsp. hot olive oil in a large skillet over medium-high heat 4 to 5 minutes or until crumbled and cooked. Proceed with recipe, folding chorizo and 2 cups cherry tomatoes, halved, into cheese sauce along with pasta in Step