2 gluten-free bread slices, toasted and broken into crumbs (optional)
1 teaspoon butter, softened (optional)
½ teaspoon paprika (optional)
Preheat the oven to 375 degrees F (190 degrees C). Grease a 9×13-inch baking dish.
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.
Melt 1/4 cup butter in a saucepan over medium heat. Stir salt and mustard powder into melted butter and remove the saucepan from heat.
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.
Stir 3 cups of Cheddar cheese into the sauce until the heat from sauce melts cheese. Add pasta to cheese sauce and stir well; pour into the prepared baking dish.
Combine remaining 1 cup Cheddar cheese, gluten-free bread crumbs, 1 teaspoon butter, and paprika in a bowl; sprinkle over pasta mixture.
Bake in the preheated oven until top is crunchy, about 30 minutes.