Bring a large pot of salted water to a boil. Add the pasta and cook as the label directs, adding the broccolini during the last 2 minutes of cooking. Drain and set aside.
Meanwhile, melt 2 tablespoons butter in a medium ovenproof nonstick skillet over medium heat. Add the sausage; cook, breaking up the meat, until lightly browned and cooked through, about 6 minutes. Remove the sausage to a plate using a slotted spoon.
Add the flour to the drippings in the skillet; cook, stirring, until incorporated, 1 minute. Whisk in the milk. Bring to a gentle simmer and cook, stirring occasionally, until slightly thickened, 3 to 5 minutes. Whisk in the cream cheese until smooth, then stir in the cheddar, cayenne and 1/2 teaspoon salt until melted and smooth, about 1 minute. Stir in the pasta and broccolini, sausage and pimentos.
Preheat the broiler. Melt the remaining 1 tablespoon butter in the microwave; stir in the panko and sprinkle the mixture over the pasta. Broil until golden brown, about 3 minutes.