Preheat the oven to 450 F (230 C).
Bake the potatoes on the middle rack. After 20 minutes, lower the oven to 400 F (205 C) and bake another 35 to 40 minutes, until very tender when pierced with a fork.
Turn off the oven. Split the potatoes lengthwise and return them to the oven cut-side up, leaving the door slightly ajar. Remove after 10 minutes, once they have dried out a bit.
Crush the amaretti into a powder using a food processor or a pestle and mortar.
Peel the potatoes and pass them through a food mill into a bowl. Add the amaretti powder, egg yolk, prosciutto, grated Parmigiano-Reggiano, parsley, a tiny grating of nutmeg (about 1/8 tsp), and salt. Mix until evenly combined.
Make yellow pasta dough by the machine method or hand-rolled method. Cut into 2-inch squares for ravioli, stuff with the sweet potato mixture, and seal. When boiling, add 1 tbsp olive oil to the water.
