Cook the pasta. Bring a 1-quart saucepan of salted water to a boil. Add the elbow macaroni and cook according to the package directions, about 7 to 8 minutes for al dente. Drain in a colander and set aside. Wipe the saucepan dry to use it again for the cheese sauce.
Make the roux. In the same saucepan over medium heat, melt the butter. Once melted, whisk in the flour and cook for 1 minute, whisking constantly. The mixture will turn pale golden and smell slightly nutty.
Add the milk. Slowly pour the milk into the roux, whisking the whole time to keep the sauce smooth. Cook for about 2 minutes, whisking often, until the sauce thickens enough to coat the back of a spoon.
Stir in the cheese. Take the saucepan off the heat. Add the shredded cheddar and dry mustard, then stir until the cheese is fully melted and the sauce is smooth.
Combine and serve. Add the cooked pasta to the cheese sauce and stir until every piece is coated. Taste and add a pinch of salt if needed. Serve right away. The sauce is creamiest when hot and thickens as it cools.
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Notes
Shred the cheese from a block. Pre-shredded cheese is coated in anti-caking agents like potato starch and cellulose that prevent it from melting smoothly. Shredding from a block is the single biggest thing you can do to get a creamy, glossy sauce instead of a grainy one.Add the milk in a slow stream. Pouring the milk in all at once is the most common cause of lumpy cheese sauce. A slow stream with constant whisking lets the roux disperse evenly into the milk before it has a chance to clump.Take the saucepan off the heat before adding the cheese. Cheese added to a hot pan can break, releasing its oils and turning the sauce greasy or grainy. Off the heat, the residual warmth is enough to melt the cheese without breaking the sauce.Serve right away. Cheese sauces thicken quickly as they cool, and the texture is best in the first few minutes. Plan to eat this recipe immediately rather than holding it on the stove or in a warm oven.If doubling the recipe, make the sauce in a 2-quart saucepan.