Heat oven to 300 degrees. Grease the bottom and sides of a 10-inch springform pan with margarine. Line the pan with parchment paper, and grease the parchment. Dust pan with 1 tablespoon potato starch; shake out excess.
Fill a small saucepan with water, and bring to a boil. Reduce heat, and allow water to simmer. Place chocolate in a small heatproof bowl, and set over saucepan. Stir chocolate until melted, and set aside to cool.
In the bowl of an electric mixer, cream 1 cup margarine on medium speed, and add sugar gradually until the mixture is pale and bubbles appear, about 10 minutes.
Add egg yolks one at a time, beating well after each addition. Lower the speed, and beat in cooled chocolate and orange juice. With a large rubber spatula, mix in cocoa powder, cognac, 1 1/2 teaspoons potato starch, lemon zest, orange zest, vanilla, and ground almonds.
In a clean dry bowl, beat egg whites on high speed until stiff. Fold half of the egg whites into the chocolate mixture with a rubber spatula. Repeat with another quarter of the egg whites, then reverse the process, pouring the batter over remaining whites. Gradually fold together until egg whites are completely incorporated.
Pour batter into prepared pan, and smooth the top. Bake until a cake tester or toothpick inserted in the center of the cake comes out slightly sticky, about 1 hour. Cool completely, and refrigerate overnight.
To serve, run a small, sharp knife around sides to loosen, and carefully remove springform ring. Transfer to a serving plate, dust with Passover powdered sugar, and garnish with sliced almonds. Note: This recipe has been adapted from Helen Nash's original to make one cake. To make two smaller cakes, prepare two 9-by-1 1/2-inch round cake pans, and divide the batter between them.