Preheat the oven to 350 degrees F. Coat a 2 or 3 quart baking dish with cooking spray.
Melt 2 tablespoons of the butter in a large pot over medium heat. Add the flour, then whisk until combined, about 30 seconds.
Pour in the water and whisk until smooth and just starting to thicken.
Add the milk and whisk until well combined.
Stir in the pasta, salt, garlic powder, onion powder, smoked paprika and pepper. Bring the mixture to a simmer.
Cook for 10-12 minutes, stirring occasionally, until pasta is tender.
Turn the heat to low, then stir in the cheeses. Keep stirring until the sauce is smooth.
Gently fold in the crab meat. Transfer the pasta mixture to the prepared baking dish.
Melt the remaining 2 tablespoons of butter, and stir it into the panko breadcrumbs.
Sprinkle the breadcrumbs over the top of the pasta. Bake for 10-12 minutes or until topping is golden brown.
Sprinkle with chives and garnish with addition crab if desired. Serve immediately.