Step 1

Preheat oven to 375 degrees F (190 degrees C). Grease a 9x13-inch baking dish.

Step 2

Bring a large pot of lightly salted water to a boil. Cook elbow macaroni in the boiling water, stirring occasionally until cooked through but firm to the bite, 8 minutes. Drain.

Step 3

Melt 1/4 cup butter in a saucepan over medium heat. Stir salt and mustard powder into melted butter and remove saucepan from heat.

Step 4

Whisk milk and cornstarch together in a bowl until smooth; stir into butter mixture until well blended. Return saucepan to stove; cook milk mixture, stirring constantly, over medium heat until sauce is thickened, about 5 minutes. Remove saucepan from heat.

Step 5

Stir 3 cups Cheddar cheese into sauce until heat from sauce melts cheese. Add pasta to cheese sauce and stir well; pour into the prepared baking dish.

Step 6

Combine remaining 1 cup Cheddar cheese, gluten-free bread crumbs, 1 teaspoon butter, and paprika in a bowl; sprinkle over pasta mixture.

Step 7

Bake in the preheated oven until top is crunchy, about 30 minutes.