Use a knife to poke the potatoes about 8 to 10 times all around.
Place the potatoes on an oven tray lined with parchment paper. Bake for 60–70 minutes or until the potatoes are soft.
Cut the potatoes in half and let the steam escape for 5 minutes.
Use a spoon to scoop out the inside of the potatoes. If they are too warm to hold, use a kitchen cloth.
Push the peeled potatoes through a sieve using the back of a spoon. Scrape the bottom of the sieve as well. If you have a potato ricer, use it instead or mash the potatoes with a fork.
Let the mashed potatoes cool down completely.
In a bowl, add the flour, a beaten egg, Parmesan, salt and pepper. Mix with a fork until combined.
Knead by hand until all ingredients are well combined.
Lightly flour the kitchen counter. Cut the dough into smaller pieces and roll each piece into a thin rope.
Cut the dough into small pillow-shaped chunks.
Roll the chunks into small balls, use enough flour to prevent sticking. Press each ball gently against the inside of a fork, then roll the edge up and fold the edges together. Place the gnocchi on a floured chopping board. Repeat until all the dough is used.
Bring a pot of water to a boil and add salt.
Heat a frying pan over medium-high heat. Add butter and let it melt.
Meanwhile, cook the gnocchi for about 90 seconds, or until they start floating.
Reserve some pasta water using a cup or ladle. Add the pasta water to the melted butter in the pan. Be careful, as it might splatter.
Tear and bruise the sage, then add it to the frying pan. Add extra Parmesan and pepper. Stir until the cheese has melted and you have a thin silky sauce.
Use a slotted spoon to add the gnocchi to the frying pan. Combine all ingredients.