Preheat your oven to 375°F (190°C). Bring a large pot of heavily salted water to a boil. Add the cauliflower florets and cook for 5-7 minutes, until tender-crisp. Drain very well and spread the cauliflower in a single layer in a 9x13 inch oven-safe baking dish.

In a large saucepan, melt 1/2 cup of unsalted butter over medium heat. Add the all-purpose flour and whisk constantly for 2 minutes to create a smooth roux.

Gradually whisk in the warmed whole milk until the mixture is smooth and lump-free. Bring the mixture to a simmer, whisking frequently, until it thickens to a consistency that coats the back of a spoon, about 5-7 minutes. Remove the pot from the heat.

Stir in the Dijon mustard, garlic powder, onion powder, black pepper, smoked paprika (if using), and 1/4 tsp salt. Add 3 cups of the shredded sharp cheddar cheese and all of the shredded Gruyère cheese, stirring until completely melted and smooth. Taste and adjust salt if needed.

Pour the prepared cheese sauce evenly over the drained cauliflower in the baking dish. Gently toss to ensure all the florets are coated in the rich sauce.

In a small bowl, combine the panko breadcrumbs with the 2 tablespoons of melted butter. Sprinkle this mixture evenly over the top of the cauliflower mac and cheese.

Bake for 25-30 minutes, or until the sauce is bubbly around the edges and the panko topping is golden brown and crispy. For an extra cheesy top, sprinkle the remaining 1 cup of sharp cheddar cheese over the dish during the last 10 minutes of baking.

Remove from oven and let stand for 5-10 minutes before serving. This allows the sauce to set slightly, making it easier to serve.