Set oven rack in upper third, and preheat to 375 degrees.
Blanch the cauliflower florets: Bring a large pot of salted water to a boil over high heat. Carefully drop the cauliflower into the water. Cook for 3 minutes. Remove and immediately put into a large bowl of ice water to fully cool. Remove from the ice water into a colander. Allow all excess water to drain and then pat dry with a towel. Roughly chop the cauliflower and place into a bowl. Set aside.
To make the Mornay sauce: In a medium pot over medium heat, melt the butter. Add the salt, garlic powder, paprika, mustard powder and black pepper. Whisk vigorously to incorporate and toast the spices. Add the flour and continue to whisk. Cook for 1 minute. Slowly add the milk while continuing to whisk, removing any lumps. Cook for 5 minutes until the sauce thickens. Reduce the heat to low and add 2 cups cheese. Stir to combine. Turn off the heat. Add the cauliflower and stir to incorporate. Season to taste.
Grease an 8x8-inch baking dish with butter. Add half of the cheesy cauliflower. Top with 1/2 cup of remaining cheese. Add the rest of the cauliflower and top with the last 1/2 cup cheese. Sprinkle paprika on top.
Bake for 25 to 30 minutes until the cheese is melted and bubbling. Remove from the oven. Set aside to cool for 10 minutes before serving.