Bring a large pot of water to a boil. Salt it generously.
Cook pasta until al dente. Before draining, scoop out ¼ cup pasta water and set aside — this starchy liquid is what makes the sauce glossy and clingy.
Drain pasta.
Return the pot to low heat. Add butter and let it melt.
Add pasta back to the pot. Toss vigorously to coat every strand in butter.
Add pasta water a splash at a time, tossing constantly, until the sauce emulsifies and coats the noodles in a light, glossy sheen — not a pool of liquid.