Preheat the oven to 350°F. Grate the chocolate and set it aside.

Place the egg whites in a large, grease-free, stainless steel or glass bowl and let them reach room temperature.

In another mixing bowl, beat the egg yolks on high speed with an electric mixer for 2–3 minutes. Gradually add 1/2 cup sugar and beat 3–4 minutes longer, until it is light and lemon-colored. Slowly add the orange juice and liqueur or vanilla. Beat 2–3 minutes longer.

Wash the beaters and dry well. Beat the egg whites until foamy. Add in the lemon juice. Increase the mixer speed to high and gradually beat in the remaining 1/2 cup of sugar. Beat until the whites are stiff and glossy, but not dry.

Sift the potato starch and salt into the egg yolk mixture. Gently fold in the whites and chocolate. Don’t worry if some white streaks remain.

Gently pour the batter into an ungreased, 10-inch tube pan. Smooth the top with a spatula. Bake the cake on the oven’s center rack for 1 hour. When it’s done, the top of the cake should be golden and will spring back when lightly touched.

Invert the pan immediately and cool completely, about 90 minutes. (If your pan doesn’t have little feet, invert it over the neck of a bottle or on a rack.)

To remove the cooled cake from the pan, slide a thin-bladed knife between the pan and the sides of the cake. Push up the bottom of the pan to loosen the sides of the cake. Then carefully remove the cake from around the center tube and the bottom of the pan. Turn the cake over onto a large, round serving platter.