Cook pasta according to package directions.
While pasta is cooking, heat butter in medium saucepan over medium heat until melted. Whisk in flour and cook until mixture is smooth. Whisk in milk and ground mustard. Cook, whisking occasionally, until well combined and hot.
Reduce heat to low. Add apple smoked cheddar, stirring until until smooth. Taste and season as needed with salt.
Combine cooked macaroni and cheese sauce and pour into 9×13-inch baking pan. Smooth into even layer.
Spread brisket over top of macaroni and cheese. Drizzle with barbecue sauce and sprinkle with blue cheese.
Set broiler to high. Slide macaroni and cheese into oven placing it 4 inches from broiler. Broil for 5-10 minutes until hot and bubbly, watching closely. Blue cheese should start to lightly brown in places.