Bring a large pot of salted water to
the boil, cook spaghetti to desired doneness.

In a large cast iron skillet, heat
olive oil over medium-high heat until shimmering. Add garlic and anchovies and
cook until fragrant, about 3 minutes. Add the broccoli rabe and continue to
cook, tossing often until the broccoli rabe is soft, about 4 minutes. Add chili
and remove from heat.

Once pasta is cooked, reserve 1 cup
of cooking liquid and then add drained pasta to the pan with broccoli rabe and
return to medium-high heat. Add cubed butter and reserved pasta water 1/4 cup
at a time, tossing pasta often until a light sauce forms. Season to taste with
salt and pepper and serve.