Preheat oven to 350 degrees F. Lightly grease a 9 x 13-inch baking dish; set aside.
Cook macaroni in a large pot of salted boiling water, according to package instructions for al dente (almost tender but still firm to the bite) -- about 6 minutes. Drain and return to the pot.
Add the cheddar, Colby and butter, stirring until the pasta is well coated.
In a separate bowl, whisk together the eggs, milk, sour cream, salt, dry mustard, garlic powder and cayenne. Add the egg mixture to the pasta and stir well to combine. Transfer the mixture to the prepared baking dish.
In a small bowl, stir together breadcrumbs and melted butter. Sprinkle over top of the macaroni.
Bake, uncovered, until golden brown and set -- about 35 minutes.
Notes
Boil the pasta in very well-salted water. The salt in the water seasons the noodles, adding flavor to the dish.
Cook the pasta just until it's al dente -- almost tender, but still firm to the bite. You don't want to completely cook the noodles until they're tender, because they will continue baking in the oven.
Allow the cheese, butter, milk, sour cream and eggs to come to room temperature before adding them to the recipe. They will blend together smoothly if they're not cold.
Start with blocks of cheese that you grate yourself at home, rather than using bags of pre-shredded cheese from the store. The pre-shredded cheese includes stabilizers and other ingredients to prevent the cheese from sticking, which inhibits its ability to melt smoothly into the dish.
If you prefer a crispier, darker brown top on your mac and cheese, place the dish under the broiler during the final minute or two of baking time. Just be sure to keep a close eye on it so that the bread crumb topping doesn't burn.
Cooking for a smaller family? Cut all of the ingredients in half and bake the casserole in an 8-inch square dish.