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1
Preheat oven to 400u00b0F (200u00b0C). In a medium pot of salted boiling water, cook elbow macaroni until just shy of al dente, about 2 minutes less than cooking time indicated on package. Drain, then transfer pasta to a large mixing bowl and toss with 2 tablespoons (30g) butter until butter is melted and pasta is evenly coated. Set aside.
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2
In a large saucepan, bring 3 cups (720ml) water to a simmer. Whisk in sodium citrate until fully dissolved. While maintaining a gentle simmer, add cheddar cheese in small increments, using an immersion blender to incorporate it completely into the sauce before the next addition. When all the cheese is added and the sauce is smooth and glossy, blend in hot sauce, mustard powder, and garlic powder. Season with salt, if necessary.
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3
Scrape cheese sauce into pasta and mix until evenly coated. Let cool slightly, then add grated Gruyere and mix well. Scrape pasta into a 9- by 13-inch baking dish and smooth surface into an even layer.
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4
Add panko to a small mixing bowl. Melt remaining 3 tablespoons butter, then add to panko and mix until evenly coated. Season with salt. Scatter panko all over surface of mac and cheese in an even layer. Bake on top rack of oven until browned and bubbling, about 45 minutes (ovens can vary; check often to prevent top from burning).
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5
Let mac and cheese rest 15 minutes, then serve. Leftover mac and cheese can be refrigerated for up to 5 days; it reheats surprisingly well in the microwave or oven.
