For the mac 'n' cheese: Bring 4 quarts of water to a boil in a large pot. Salt generously, and then add the pasta and cook to box instructions, usually 6 to 7 minutes. Drain.
In another large pot, melt the butter over medium-high heat. Whisk in the flour and continue whisking until it deepens in color and releases a light toasted aroma, about 1 minute.
Whisk in the Thai red curry paste, and when fully combined, pour in the milk and cream (or all milk, if using). Bring to a boil, whisking constantly, and then turn down to a simmer. Simmer until the mixture thickens, whisking occasionally, about 5 minutes.
Off heat, whisk in the cheeses and stir until melted. Add the pasta and toss to combine.
Turn your broiler on.
For the crumb topping: Toss the panko with the melted butter and season with salt and pepper.
To assemble: Pour the mac 'n' cheese into a broiler-safe casserole dish, sprinkle with the bread crumb mixture, and broil until browned, about 1 minute.