1. Cook macaroni according to package directions; drain well.
2. Preheat oven to 350°F. Mist a 2 1/2-quart baking dish with cooking spray.
3. Warm milk in a large saucepan over medium heat. Gradually add flour, whisking steadily until combined. Add mustard, garlic powder, salt, pepper and Worcestershire sauce. Continue cooking until sauce thickens, stirring constantly.
4. Reduce heat to low. Whisk in cream cheese until well blended; add 1 1/3 cups shredded Cheddar and stir until melted. Add macaroni, stirring until well coated. Pour into baking dish.
5. Toss remaining Cheddar with cornflake crumbs and sprinkle evenly over top.
6. Bake until golden on top and bubbling throughout, about 20 minutes.