In a large heatproof bowl, combine butter and cheese.
In a medium pot of salted boiling water, cook pasta until al dente.
Using tongs, a strainer, or a spider, transfer pasta to bowl with cheese and butter.
Add ½ cup hot pasta-cooking water, then, using tongs, toss repeatedly until butter is melted and a creamy, emulsified sauce forms.
If pasta seems too dry, add a splash or two of additional pasta water until desired consistency is reached.
Season with salt, if necessary.
Serve, sprinkling more cheese on top at the table.
