How to Make It
In a small saucepan, cover the potatoes with cold water and bring to a boil. Cook until tender, about 5 minutes. Drain and set aside.
At the same time, fill a stockpot with 6 quarts of water, cover and bring to a boil. Add 3 1/2 tablespoons salt to the water and add the spaghettini. Cover partially until the water just returns to a boil, then uncover, stir the pasta and cook until al dente. Drain, reserving 1/4 cup of the pasta water.
Meanwhile, in a serving bowl large enough to hold the spaghettini, mash the Gorgonzola with the butter and sage with the back of a spoon. Add the spaghettini and potatoes, season with pepper and toss gently but thoroughly. Add 1 or 2 tablespoons of the pasta water if necessary. Serve immediately.