Pour the evaporated milk in a medium saucepot and bring to a simmer over medium heat. Toss together the grated cheeses and cornstarch in a bowl until evenly coated, then add the mustard. Add the cheese mixture to the simmering evaporated milk and whisk constantly, being careful not to scald or scorch the bottom of the pot, until homogeneous and thickened, about 10 minutes. Season to taste with salt and pepper. Reserve 1 1/2 cups of cheese sauce in a heatproof measuring cup.
Add the sauce to the macaroni and season to taste. Reserve 1 cup mac 'n' cheese and stir the remaining mac 'n' cheese with the beaten eggs.
Spray the inside of a Bundt pan with a liberal amount of cooking spray. Working quickly, spread the macaroni mixture into the pan and bake until golden brown, about 25 minutes. Let cool 10 minutes.
Microwave the reserved cup cheese sauce (or reheat gently on the stovetop) until pourable, 2 to 3 minutes, then add the orange food coloring until the desired orange color is achieved.
Using a thin spatula, loosen the mac 'n' cheese from the sides of the Bundt. Turn out on a dish and fill the hole with the reserved cup mac 'n' cheese. Cover with the remaining orange cheese sauce. For extra effect, add the broccoli stem in the center. Serve immediately!