To cook the gnudi, heat a large, wide pan with boiling, salted water. It’s easiest to cook the gnudi in two batches so they don’t stick together. Turn the heat down so the water is at a gentle simmer, then lower in the gnudi. Simmer for 3 minutes, or until they float to the surface, then use a slotted spoon to scoop them into the butter pan. Repeat with the second batch, then put the butter pan back on the heat and just warm the butter through before spooning over the gnudi. Serve in warm bowls with the sage and extra parmesan, if you like.