1. Cook the pasta and potatoes: Bring a large pot of well-salted water to a boil. Add the diced russet potatoes and cook for 4–5 minutes, just until barely tender. Add the pasta to the same pot and cook according to package directions until al dente. Drain the pasta and potatoes together and set aside.

2. Cook the bacon: While the pasta cooks, place the bacon in a large skillet over medium heat. Cook until crispy, then transfer to a paper towel–lined plate to drain. Once cool, crumble the bacon and reserve 1–2 tablespoons of the bacon drippings if desired for extra flavor.

3. Make the roux: In a large saucepan over medium heat, melt the butter (along with the reserved bacon drippings, if using). Sprinkle in the flour and whisk constantly for 1–2 minutes, until the mixture is smooth and lightly bubbling but not browned.

4. Build the cheese sauce: Slowly pour in the whole milk while whisking, then add the heavy cream. Continue whisking until the sauce thickens and coats the back of a spoon, 4–6 minutes. Stir in the garlic powder, onion powder, smoked paprika, salt, and black pepper.

5. Add the cheeses: Remove the saucepan from the heat. Stir in 2 cups of cheddar and 1 cup of mozzarella until completely melted and the sauce is smooth and creamy.

6. Combine in the baking dish: Lightly grease a 9×13-inch baking dish. Add the drained pasta and potatoes, then pour the hot cheese sauce over the top. Fold gently until everything is evenly coated. Stir in the sour cream and about half of the crumbled bacon.

7. Make the topping: In a small bowl, combine the panko bread crumbs, remaining 0.5 cup cheddar, remaining 0.5 cup mozzarella, and olive oil or melted butter. Toss until the crumbs are evenly moistened.

8. Assemble and bake: Sprinkle the panko mixture evenly over the mac and cheese, then scatter the remaining bacon on top. Bake at 375°F (190°C) for 20–25 minutes, or until the casserole is bubbly and the topping is golden brown.

9. Finish and serve: Let the dish rest for 5–10 minutes so it can set slightly. Top with extra sour cream, sliced green onions, and chives. Scoop into bowls and serve warm.

10. Enjoy leftovers: Store cooled leftovers in airtight containers in the fridge for up to 4 days. Reheat gently with a splash of milk to keep the sauce creamy.