Place butter and garlic powder in a microwave safe dish and microwave for a few seconds until butter is melted; stir to combine garlic powder into melted butter.
Brush butter on the inside of each piece of mini sub roll and place on cookie sheet, buttered side up.
Broil in oven for a few minutes until lightly brown and toasty.
Remove from oven and transfer mini sub tops to serving plates.
Divide Mozzarella cheese among the 4 bottoms of the mini sub rolls and place back under the broiler until cheese melts (watch them closely; don’t let them burn!).
Remove from oven and place on serving plates beside the mini sub roll tops.
In the meantime, heat sauce in microwave-safe dish in microwave for a few minutes.
Also, place meatballs in a microwave-safe dish and microwave them, too, for a few minutes.
Assemble each sandwich by placing 2 meatballs on each sub roll, top each meatball with a tablespoon of sauce, garnish with basil and serve.