Step 1: Preheat oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
Step 2: Cook elbow macaroni according to package directions until al dente. Drain well and set aside.
Step 3: In a large Dutch oven or heavy-bottomed pot, cook the chopped bacon over medium heat until crispy. Remove the bacon with a slotted spoon, transferring it to a paper towel-lined plate. Reserve 2 tablespoons of bacon fat in the pot.
Step 4: Add 1/2 cup unsalted butter to the reserved bacon fat in the pot. Once melted, whisk in the flour and cook for 1-2 minutes, stirring constantly, to create a smooth roux.
Step 5: Gradually whisk in the warmed whole milk until smooth. Then, slowly whisk in the Oktoberfest beer until fully incorporated and the sauce begins to thicken slightly.
Step 6: Stir in the Dijon mustard, smoked paprika, garlic powder, and cayenne pepper (if using). Season generously with salt and freshly ground black pepper. Bring the sauce to a gentle simmer, then reduce heat to low.
Step 7: Remove the pot from the heat. Add the shredded sharp cheddar and Gruyère cheeses in small handfuls, stirring continuously until each addition is fully melted and smooth before adding more. This prevents the cheese from seizing.
Step 8: Gently fold the cooked macaroni and about half of the crispy bacon into the cheese sauce until evenly coated.
Step 9: Pour the mac and cheese mixture into the prepared baking dish.
Step 10: In a small bowl, combine the Panko breadcrumbs and 2 tablespoons of melted butter. Sprinkle this mixture evenly over the top of the mac and cheese, along with the remaining crispy bacon.
Step 11: Bake for 20-25 minutes, or until the mac and cheese is bubbly and the topping is golden brown and crispy.
Step 12: Let the dish stand for 5-10 minutes before serving. Garnish with fresh chives or parsley, if desired.