Cook the macaroni following package directions. Drain thoroughly and set aside.
Meanwhile, melt 4 tablespoons of butter in a medium saucepan over medium heat. Add the flour, dry mustard, salt, pepper, and garlic powder, if using. Cook for 2 minutes, stirring constantly. Add the milk and beer. Cook until thickened and simmering, stirring constantly. Stir in the shredded cheddar cheese and continue to cook until cheese has melted.
Combine the sauce and the drained macaroni.
Spoon the macaroni mixture into the prepared baking dish.
Melt the remaining 2 tablespoons of butter and toss with the crushed pretzels. Sprinkle the buttery pretzel crumbs over the casserole.
Bake in the preheated oven for 25 to 30 minutes, until hot and bubbly.