Preheat oven to 350 degrees.
Butter and flour the cake pan. Set the chocolate and rum or coffee in a small pan, cover, and place (off heat) in a larger pan of almost simmering water; let melt while you proceed with the recipe. Measure out the rest of the ingredients.
Cream the butter and 2/3 cup of sugar together for several minutes until they form a pale yellow, fluffy mixture. Beat in the egg yolks until well blended.
Beat the egg whites and salt in a separate bowl until soft peaks are formed; sprinkle 1 Tbsp. sugar and beat until stiff peaks are formed.
With a rubber spatula, blend the melted chocolate into the butter and sugar mixture, then stir in the almonds, and almond extract. Immediately stir in one fourth of the beaten egg whites to lighten the batter. Delicately fold in a third of the remaining whites and when partially blended, sift on one third of the flour and continue folding. Alternate rapidly with more egg whites and more flour until all egg whites and flour are incorporated.
Turn the batter into the cake pan or into mini cake pans, pushing the batter up to its rim with a rubber spatula. Bake in middle level of preheated oven for about 25 minutes (20 minutes for very small mini cake pans). Cake is done when it has puffed, and 2 1/2 to 3 inches around the circumference are set so that a needle plunged into that area comes out clean; the center should move slightly if the pan is shaken, and a needle comes out oily.
Allow cake to cool in the pan for 10 minutes. Run a knife around the edge of the pan, and reverse cake on the rack. Allow it to cool for an hour or two; it must be thoroughly cold if it is to be iced.