Preheat oven to 425º F. Line a baking sheet with nonstick foil or spray with nonstick cooking spray.
Place broccoli florets in a gallon size resealable bag, drizzle with olive oil, shake to coat. Add panko bread crumbs, parmesan cheese, and Italian seasoning; shake to coat. There will be loose crumbs in the bag, which will be sprinkled on top of the finished mac and cheese.
Dump broccoli and crumbs onto the prepared baking sheet. Season with Kosher salt and freshly ground black pepper. Roast for 10-15 minutes, watching closely so the crumbs don’t burn.
While broccoli is roasting, add the macaroni, butter, salt, dry mustard, cayenne, garlic powder, cornstarch, 2 ½ cups of water, and only 2 cups of milk to the pressure cooker pot. Stir well, to incorporate cornstarch.
Secure the lid and turn pressure release knob to a sealed position. Cook at high pressure for half of the pasta package cooking time minus 1 minute. (For example, if cook time is 16 minutes, cook for 7 minutes. If it says 12-13, cook for 6).
When cooking is complete, use a quick release. If liquid sprays from valve, close valve, wait 30-60 seconds and try again. If pasta isn’t done, use sauté function to cook with the lid off for 2-3 more minutes.
Select sauté function, add ½ cup of milk (adding another ½ cup, if needed), cheddar, and Quesadilla cheese; stir until melted and creamy.
Serve immediately, topped with the roasted broccoli and extra crumbs.