Heat oven to 400 °F.
Line rimmed baking sheet with parchment paper. Arrange cauliflower in a single layer. Drizzle lightly with olive oil. In small bowl, mix together 1/2 teaspoon paprika, chipotle chile powder, garlic powder and 1/2 teaspoon salt. Sprinkle evenly over cauliflower; roast 10 to 15 minutes, or until lightly browned. Set aside.
Melt 2 tablespoons butter in skillet over medium heat. Add leeks; sauté until softened, about 10 minutes. Set aside.
Reduce oven temperature to 375°F. Cook pasta according to package directions for al dente. Drain and set aside.
In large sauté pan or Dutch oven, melt remaining 4 tablespoons butter over medium heat. Whisk in flour and mustard powder, stirring continuously until blended and lump- free. Stir in heavy cream (or milk) and remaining 1/2 teaspoon paprika. Season with salt and pepper. Reduce heat to low and cook 5 minutes, until thickened.
Stir in 3/4 cup cheddar, 1 cup fontina and 1 cup butterkäse cheese. Add pasta, cauliflower and leeks, mixing well. Transfer to prepared 9x13-inch casserole dish.
Top evenly with remaining 3/4 cup cheddar; bake 25 to 30 minutes, or until cheese is melted and bubbly. Let sit 5 minutes before serving.