01 - Preheat oven to 400 degrees Fahrenheit. Line a large baking sheet with parchment paper.
02 - For each taco shell, sprinkle 1/4 cup of Parmesan cheese into a 5-inch circle on the baking sheet. Repeat to create eight circles spaced evenly apart.
03 - Bake the cheese circles in the preheated oven for 5 to 7 minutes, until golden and bubbling. Remove immediately from the oven and, while hot, drape each over a wooden spoon handle or the edge of a bowl to shape into taco shells. Allow to cool and set completely.
04 - Bring water to a boil and cook elbow macaroni according to package instructions until al dente. Drain thoroughly and set aside.
05 - In a medium saucepan over medium heat, melt butter. Add flour and whisk continuously for 1 minute to develop a roux.
06 - Gradually whisk in whole milk, stirring until the mixture is smooth and slightly thickened, about 2 to 3 minutes.
07 - Lower heat, add shredded cheddar, mozzarella, mustard powder, garlic powder, ground black pepper, and salt. Stir until all cheese has melted and sauce is smooth.
08 - Add the drained macaroni to the cheese sauce, folding gently until the pasta is evenly coated.
09 - Carefully spoon the mac and cheese filling into each cooled Parmesan shell.
10 - Top each taco with sliced green onions, diced tomatoes, jalapeños if desired, and chopped parsley. Serve immediately while shells are crisp.