Preheat oven to 350 degrees F. and grease a 10-inch springform pan. Melt chocolate and butter over a pot of barely simmering water until smooth. Stir in egg yolks gently, one at a time, then fold in orange zest and ground almonds. Whip egg whites until foamy, then add sugar while whipping until whites hold a medium peak. Fold whites into chocolate mixture in 2 additions and gently fold in orange liqueur and vinegar (the vinegar keeps the cake from cracking too much as it rises). Pour batter into prepared pan and bake for 35 to 45 minutes, until a tester inserted in the center of the cake comes out clean. Let cake cool completely in pan. Remove cake from pan and dust with icing sugar and/or cocoa powder.