Preheat the oven to 375°F and grease a mini muffin pan well with nonstick spray.
Cook the elbow macaroni in salted boiling water until just tender. Drain well.
In a saucepan, melt the butter over medium heat. Whisk in the flour and cook for 1 minute.
Slowly whisk in the milk and cook until the sauce thickens.
Stir in the cheddar, mozzarella, salt, pepper, and garlic powder until melted and smooth.
Add the cooked macaroni and stir to coat evenly. Let the mixture cool slightly.
Stir in the beaten egg until fully combined.
In a small bowl, mix the panko breadcrumbs with 1 tablespoon melted butter.
Spoon the mac and cheese mixture into the mini muffin cups, pressing lightly.
Top with the extra cheddar and buttered breadcrumbs.
Bake for 18 to 22 minutes, or until golden on top and set around the edges.
Cool in the pan for 8 to 10 minutes before gently removing. Serve warm.