How to Make It
Mash softened butter with cheese, black pepper, and 1 teaspoon kosher salt. Roll butter in parchment; freeze until firm.
Cook broccoli rabe in a large pot of boiling salted water until crisp-tender, 2 to 3 minutes. Using a slotted spoon, transfer to a colander to drain. Add pasta to boiling water and cook until al dente. Reserve 1/2 cup cooking water; drain pasta.
In a serving bowl, toss pasta with broccoli rabe. Using a box or hand grater, grate some compound butter over pasta and toss vigorously to coat. Season with salt and toss again. Serve immediately.