Preheat an oven to 180 degrees C. Grease a 20x30cm baking dish.
Place the chickpeas into a blender. Cover and puree until smooth. Scrape the puree into a small bowl and stir in the bread crumbs and Parmesan cheese; set aside.
Fill a large pot with lightly salted water and bring to a rolling boil over high heat. Once the water is boiling stir in the macaroni and return to a boil. Cook the pasta uncovered, stirring occasionally, until the pasta has cooked through but is still very firm to the bite, about 5 minutes. Drain well in a colander set in the sink, and rinse with cold water until cold; set aside.
Meanwhile melt 2 tablespoons of butter in a large saucepan over medium-low heat. Whisk in the flour and stir until the mixture becomes paste-like and light golden brown, about 5 minutes.
Gradually whisk the evaporated milk and water into the flour mixture and bring to a simmer over medium heat. Season to taste with chilli flakes, nutmeg, salt and pepper. Cook and stir until the mixture is thick and smooth, 10 to 15 minutes.
Once the sauce has thickened whisk in the sweet potatoes and sour cream; return to a simmer. Whisk in half of the Cheddar cheese until melted. Stir in the macaroni until well coated with the sauce then pour half into the prepared baking dish. Sprinkle the remaining cheese on top then cover with the remaining macaroni. Smooth the top then dot with the chick pea mixture and the remaining 2 tablespoons of butter.
Bake in the preheated oven until the sauce is bubbly and the top is golden brown, 30 to 40 minutes. Allow the dish to rest for 5 minutes before serving.