Preheat the oven to 350 degrees F. Line a 9-inch springform cake pan with foil.
In a bowl with an electric mixer cream the butter with all but 1/4 cup of the sugar and beat until light and fluffy. Add the egg yolks, one at a time, and beat the mixture until combined well. Stir in the chocolate, almonds, cognac, potato starch, cinnamon and salt.
In a bowl with an electric mixer beat the egg whites until they are foamy. Add the remaining sugar, a little at a time, and beat the whites until they form soft peaks. Stir 1/4 of the whites into the batter and fold in the remaining whites gently but thoroughly.
Pour the batter into the prepared pan and bake for 40 minutes, or until puffed and still slightly loose in the center Transfer to a rack to cool completely.
To unmold: release the sides from the pan and peel back foil. Invert cake onto a plate and carefully peel off foil from bottom of cake. Invert cake onto a serving plate and dust top with confectioner's sugar.