1. Preheat the oven to 350 degrees Fahrenheit (180 degrees Celsius). Grease a tall 9-inch round cake pan or springform pan. Line the bottom of the pan with a circle of Gefen Parchment Paper and grease the parchment paper.
2. Melt the chocolate and margarine over a double boiler. Remove from the heat and let cool slightly. Set aside.
3. Beat the egg yolks with the brown sugar on medium speed until very pale, thick, and fluffy, about three minutes. Reduce the speed, add the chocolate mixture, and mix just to combine. Add the flour, mixing just to combine. Blend in the liqueur and vanilla. Transfer to a large mixing bowl and set aside.
4. In a clean mixing bowl with clean beaters, beat the egg whites with the salt on high speed until they hold soft peaks, one to two minutes. With a rubber spatula, fold one third of the egg whites into the chocolate mixture to lighten it, and then gently fold in the remaining whites. Pour the batter into the prepared cake pan.
5. Fill a roasting pan halfway with water and place on the rack below where your cake will be baking. Put cake in the oven and bake until the top feels set, 40–45 minutes. Remove the cake from the oven and run a paring knife around the inside of the pan to loosen the cake, then let the cake cool completely in the pan on a rack.
6. When the cake is completely cool, loosen the sides once more with a paring knife. Cover the cake with a serving plate and invert the cake onto the plate. The bottom of the cake is now the top. Peel off the parchment paper.