1.Cook potatoes in a large pot of boiling water over medium-high heat until tender and drain.
2. As soon as potatoes are cool enough to handle, peel and pass through a potato ricer into a large bowl.
3. Sprinkle 1¼ cups flour and 2 tsp. salt over potatoes and, using your hands, make a well in the center.
4. Pour egg into the well and stir in with a wooden spoon.
5. Turn out dough onto a floured surface and gently knead, dusting with more flour as needed, until smooth but not elastic (be careful not to overwork)
6.Divide dough into 8 pieces. Roll each piece into a 24″-long rope about ½” thick. Cut into ½” pieces, dust with flour, and arrange in a single layer on a lightly floured rimmed baking sheet.
7. Toss butter, Parmesan, and sage in a large bowl. Working in batches, cook gnocchi in large pot of boiling salted water until they float to the surface.
8.Using a slotted spoon, gently place on top of butter mixture as you go. Once all gnocchi are cooked, add ½ cup cooking liquid and gently toss everything together, adding more cooking liquid as needed, until butter and cheese are melted and a creamy sauce forms.
9.Serve gnocchi topped with pepper and more Parmesan.