Preheat oven to 180°C (355°F). Place the potatoes on a baking tray and bake for 1 hour or until soft in the middle and crispy on the outside. Allow to cool slightly. Cut in half, scoop the flesh into a bowl and mash until smooth. Add the parmesan, flour, salt and pepper and stir to combine. Add the egg yolks and stir until the mixture comes together and leaves the sides of the bowl. Turn out onto a lightly floured surface and knead for 1 minute or until a soft dough forms. Divide dough in half and roll each piece into 64cm lengths. Cut into 2cm pieces and place on a lightly floured tray. Cook the gnocchi, in batches, in a large saucepan of salted boiling water for 3–4 minutes or until they float to the surface and are firm. Remove with a slotted spoon and keep warm.
Heat a frying pan over high heat. Add the butter and sage and cook for 3–4 minutes or until the sage is crispy and the butter just turns brown. Add the lemon juice, salt and pepper. Divide the gnocchi among serving bowls and spoon over the butter. Sprinkle with extra parmesan to serve. Serves 4.