Preheat oven to 350°F. Cook macaroni according to package until al dente; drain and set aside.
Melt 6 tablespoons butter in large pot over medium; whisk in flour and cook 1-2 minutes, stirring constantly, until golden and bubbly.
Gradually whisk in both milks; stir constantly about 5-7 minutes until thickened. Add salt, pepper, and garlic powder.
Remove from heat; stir in cheeses until completely melted and smooth. Fold in macaroni to coat well.
Pour into greased 9x13-inch dish. Combine panko and melted butter; sprinkle evenly over top. Bake 25-30 minutes until bubbly and golden brown on top.