Boil a pot of water and cook the elbow macaroni according to package instructions
Preheat the broiler
Heat the margarine in a skillet or saucepan over medium heat until it sizzles.
Add the flour and stir it around with the butter. Cook the flour for about 2 minutes.
Slowly whisk in the milk. Whisk well as you add the milk to avoid lumps!
Whisk in the garlic powder, dry mustard and black pepper. Keep whisking until the milk mixture begins to boil and thicken.
Turn the heat down and whisk in the cheese until the cheese is melted and fully incorporated into the sauce.
Remove the sauce from heat and stir in the nonfat Greek yogurt.
Stir in the cooked, drained elbow macaroni
Spoon the macaroni and cheese mixture into 8 small ramekin cups. Sprinkle with about ½ tablespoon of panko breadcrumbs each. Spray lightly with nonstick spray or an olive oil mister.
Arrange the ramekins on a baking sheet and place under the broiler for about 5 minutes, until the breadcrumbs on top are golden.