01 - Preheat oven to 400°F. Line a large baking sheet with parchment paper.
02 - Sprinkle ¼ cup of grated Parmesan into a 5-inch circle on the prepared baking sheet for each taco shell. Repeat to make 8 circles, leaving space between each.
03 - Bake Parmesan cheese circles for 5 to 7 minutes until golden and bubbling. Remove from oven.
04 - While hot, carefully drape each Parmesan disc over a thick wooden spoon handle or the edge of a bowl to form a taco shell shape. Let cool completely to set.
05 - Boil elbow macaroni in salted water according to package directions. Drain and set aside.
06 - In a medium saucepan, melt butter over medium heat. Whisk in flour and cook for 1 minute.
07 - Gradually whisk in whole milk, stirring constantly, until the mixture is smooth and thickened, about 2 to 3 minutes.
08 - Lower heat. Add shredded cheddar, mozzarella, mustard powder, garlic powder, black pepper, and salt. Stir until cheese is melted and sauce is velvety.
09 - Transfer drained macaroni into the cheese sauce. Mix thoroughly until evenly coated.
10 - Carefully spoon prepared mac and cheese into each Parmesan taco shell.
11 - Top filled tacos with green onions, diced tomatoes, jalapeños if desired, and chopped parsley. Serve immediately.