Preheat the oven to 375°F (190°C) and grease a 9x13-inch baking dish.

Bring a large pot of lightly salted water to a boil. Cook gluten-free elbow pasta for 8 minutes, stirring occasionally until firm to the bite. Drain.

In a saucepan, melt ¼ cup butter over medium heat. Stir in salt and mustard powder, then remove from heat.

Whisk together milk and cornstarch in a bowl until smooth. Add mixture to the butter mixture in the saucepan and cook over medium heat, stirring constantly until thickened (about 5 minutes). Remove from heat.

Stir 3 cups of shredded Cheddar cheese into the sauce until melted. Combine the cooked pasta with the cheese sauce and transfer it to the prepared baking dish.

In a bowl, combine the remaining 1 cup of shredded Cheddar cheese, gluten-free bread crumbs, 1 teaspoon of softened butter, and paprika. Sprinkle over the pasta mixture.