Bring a large pot of salted water to a rolling boil and cook pasta according to package directions. Drain and set aside.
In a large skillet over medium heat, cook butter, gently shaking the pan occasionally, until it foams and turns nut-brown, about 3 minutes.
Add sage and red-pepper flakes; then cook 30 seconds longer, or until sage is crisp. Remove pan from heat.
Add cooked pasta to skillet; then toss with butter sauce to coat. Add salt and pepper to taste.
Garnish with fresh sage leaves. Serve immediately.