Preheat the oven to 350°F. Lightly butter a 9- by 13-inch baking dish.
In a medium saucepan over moderate heat, melt 3 tablespoons of the butter.
Put the panko bread crumbs in a small bowl, drizzle with the melted butter, and toss to combine. Add 1/4 of Parmesan cheese to the bowl, toss to combine, and set aside.
In a large pot of boiling salted water, cook the elbow macaroni until al dente. Drain the noodles, without rinsing, then return them to the pot.
In a small saucepan over moderate heat, warm the milk just until it starts to steam and form tiny bubbles around its edges. Remove from the heat.
In the same medium saucepan over moderate heat, melt the remaining 3 tablespoons of butter. Saute the minced garlic for 1-2 minutes.
Add the flour. Use a flat-edged wooden spoon to stir the mixture together, scraping the bottom of the pan to prevent burning, just until it starts to turn a light brown color, about 3 minutes. This will "cook" out the flour flavour.
Slowly add the warm milk and stir constantly until the sauce thickens, about 5 minutes. Remove from the heat and stir in the dry mustard, salt, and pepper.
Add the Cheddar and Parmesan cheeses. Stir until it's completely melted. Adjust the seasoning to taste.
Pour the cheese sauce over the cooked elbow macaroni and gently stir to coat the noodles in the sauce.
Pour the macaroni and cheese into the prepared baking dish and top with the panko-cheese mixture. Bake until light golden brown and bubbling, about 30 minutes. Serve hot.