Preheat oven to 350 degrees Fahrenheit.

Using the steel blade of a food processor, break chocolate into one-inch chunks. Process until fine, about 30 seconds. Add almonds (if using). Process until almonds are finely chopped, 12 to 15 seconds longer.

In a large mixing bowl, beat egg yolks with an electric mixer until light, three to four minutes. Add sugar and water and beat on high speed for 8 to 10 minutes. Combine potato starch and cake meal. Sprinkle over yolk mixture a little at a time (a sifter or strainer will help) and fold in carefully. Then fold in grated chocolate and nuts. Wash beaters thoroughly and dry well.

In another large mixing bowl, beat egg whites with salt until stiff but not dry. Carefully fold into batter. Pour gently into an ungreased 10-inch tube pan. Batter should come to within one and a half inches of the top of the pan. If necessary, make a two-inch collar with foil around top of pan.

Bake for one hour, then reduce heat to 300 degrees for 15 minutes. Invert immediately and cool completely.