Place a rack in the top third of the oven and preheat to 375°F. Mist a 2-quart baking dish with cooking spray. Bring a large pot of salted water to a boil and cook pasta until al dente, about 7 minutes or as package label directs. Drain and place back in pot.
In a pan over medium-high heat, bring milk and squash to a simmer, stirring occasionally, about 5 minutes. Remove from heat and add mustard, 1/2 tsp. salt, cayenne and Cheddar, stirring until cheese has melted. Stir sauce into pasta and toss until pasta is well-coated. Transfer mixture to baking dish.
In a small bowl, mix butter, panko and Parmesan; sprinkle over macaroni. Bake until bubbling around edges and beginning to brown on top, about 20 minutes. Turn broiler to high and cook until top is golden and crispy, about 2 minutes, watching carefully to be sure it doesn't burn. Let rest for 5 minutes before serving.
This recipe was previously published in All You, May 2013.