Step 5: Stuffing:

Stuffing the twirled pasta balls:

While the meatball mixture is resting,  remove the pasta balls from the freezer.

Cut the mozzarella cheese into small cubes and push/work the cubes into both sides of the twirled pasta ball. 

Repeat until all pasta balls are stuffed with cheese.

Put the pasta balls back into the freezer until the meatball mixture has finished chilling.

Stuffing the meatballs:

Scoop out about 2-3 Tablespoons of meat mixture into your palm.  

Flatten it slightly and push the pasta ball inside.

Work the meat mixture gently around the pasta/cheese ball to seal it in.  Gently roll the meatballs into shape.  If you pack it too hard, the cheese will explode during baking. ;-(

Cover and chill the stuffed meatballs for at least 30 minutes before baking.  They can be kept refrigerated up to 24 hours if you want to make them ahead of time.