Preheat the oven to 350 degrees F. Grease the sides and bottom of a 10 x 2- 1/2 inch springform pan. Dust with flour or potato starch, then invert the pan and tap to shake out the excess. Place the chocolate in the top of a double boiler, cover and set over simmering water until melted. Let cool. In the bowl of an electric mixer beat the egg yolks at medium speed for 5 minutes, gradually adding 1/2 cup of sugar. Beat in the chocolate and the rum. Set aside. In a clean dry bowl beat the egg whites at high speed until they are barely foamy; gradually add the remaining sugar and beat until stiff. With a large rubber spatula, fold 1/2 of the walnuts and and 1/4 of the whites into the batter. Repeat with the remaining walnuts and another 1/4 of the whites. Then reverse the process, pouring the batter over the whites. Gradually fold the 2 batters together, making a motion like a figure eight, until all the whites have disappeared. Pour the batter into the prepared pan and smooth the top. Bake for 40 minutes, or until a cake tester or a toothpick inserted in the center of the cake comes out clean. Cool on a wire rack. Loosen the sides of the cake with a knife and remove the rim of the pan. To Serve: Loosen the bottom of the cake with a knife and invert the cake onto a serving platter. NOTE: "This is a large chocolatey, moist cake that is easy to make. It is ideal for Passover or any large party. It stays fresh for several days refrigerated, or you may freeze it." (Helen Nash) Helen Nashs Kosher Kitchen by Helen Nash Random House Inc, NY. 1988 p. 244 ISBN 0-394-57026-x Recipe by: Helen Nashs Kosher Kitchen p. 244 Posted to JEWISH-FOOD digest by Linda Shapiro on Aug 11, 1998,