In a small bowl, combine butter and garlic powder until incorporated. Spread the butter on the cut sides of the buns. Place the buns cut side up on a baking sheet under the broiler set to High. Cook for about 1 to 2 minutes, keeping a close eye on them, until toasted. Remove from oven and set aside.

Preheat oven to 400˚F.

Add beef to the hoagie buns, dividing the meat evenly between them. Top with provolone slice per sandwich. Close the buns and wrap the sandwiches in a layer of aluminum foil each. Place in the oven to warm for 10 minutes.

While the sandwiches are in the oven, warm the au jus over the stove on medium heat until hot.

Serve sandwiches with au jus for dipping.