Preheat the oven to 350°F. Lightly grease a 9x9-inch (or 2-1/2 quart) baking pan with cooking spray and set aside.
Bring a large pot of water to boil, then generously salt the water; I add 1 tablespoon. Once boiling, add pasta and cook for 1 minute less than the package directs. As soon as the pasta is done, drain and return to the empty pot. Add 1 tablespoon butter and gently stir until butter is melted. Set aside to cool slightly.
In a medium pot, melt 4 tablespoons butter on medium heat . Once melted, add in garlic powder, onion powder, mustard powder, salt, and pepper. Whisk, then add the flour and whisk again until smooth. Cook, stirring constantly, for 1 minute.
Very gradually, while whisking constantly, add in the evaporated milk. Briskly whisk as you add the milk until the sauce is smooth. Once smooth, cook, whisking frequently until mixture is thickened nicely, about 5–7 minutes (see note 4). Once thickened, remove from heat. Add in the grated cheeses and stir—it’s ok if cheese isn’t fully melted, since we’re baking it all!
Add the last 2 tablespoons of butter to a skillet. Heat to medium, and once melted, add in the panko and another sprinkle of salt, about 1/4 teaspoon. Gently stir until butter is absorbed and panko is toasted to a light brown color. Remove from heat.
Use a spatula to scrape every bit of sauce onto the cooked elbows. Gently stir—it’s going to be very saucy; you don’t want dry pasta after baking. Sprinkle Parmesan cheese on top. Sprinkle panko topping evenly. Bake for 10–15 minutes or until the top is a light golden brown—don’t bake too long or it will dry out the pasta!
Notes
Note 1: Evaporated milk adds great flavor and creaminess while thickening quickly. Whole milk is the best substitute; I would not recommend lower-fat or non-dairy options. For lactose-free, use whole Lactaid® milk.Note 2: I think I’ve tested just about every combo out there, and the best blend is equal parts sharp Cheddar, smoked gouda, and gruyere. For two-cheese options, try 2 cups gruyere with 1 cup Cheddar, 2 cups Cheddar with 1 cup gouda, or 2 cups gruyere with 1 cup mozzarella. Be sure to shred your own cheese; pre-shredded cheese will give the sauce a powdery texture and won’t melt as well.Note 3: Panko is a type of breadcrumb that creates an amazing crunch, but plain breadcrumbs will work.Note 4: The sauce should be thick enough to coat the back of a wooden spoon. Lift up the spoon and immediately trace a line across the back of the spoon with your fingertip. If the line retains a clear track, the sauce is thick enough. If not, the sauce needs to be cooked a bit longer.Storage: This recipe is best enjoyed straight from the oven. Store leftovers in an airtight container in the fridge. To reheat, microwave, then lightly spray the topping with cooking spray before broiling in the oven just long enough to recrisp.