For the egg whites, add a pinch of salt and beat on high until soft peaks form. Add sugar and continue to beat until peaks stiffen.
Preheat oven to 350 degrees F. Butter and flour cake pan. Set chocolate and rum in a bowl set over simmering water to melt. Remove from heat when almost completely melted.
Cream butter and 2/3 cup sugar together for several minutes until they form a pale yellow fluffy mixture. Beat in egg yolks until well blended.
Beat in egg whites and salt in separate bowl until soft peaks are formed; sprinkle on 1 tbsp sugar and beat until stiff peaks are formed. With spatula, blend melted chocolate into butter and sugar mixture, then stir in almonds, and almond extract.
Immediately stir one fourth of egg whites to lighten the batter. Delicately fold in a third of remaining whites and when partially blended, sift on one third of flour and continue folding.
Alternate rapidly with more egg whites and more flour until all egg whites and flour are incorporated. Turn batter into cake pan, pushing batter up to its rim with a spatula. Bake in middle level of oven for about 25 minutes. 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; center should move slightly if pan is shaken, and a needle comes out oily.
Allow cake to cool in pan for 10 minutes. Run knife around edge of pan, and reverse cake on rack. Allow it to cool for an hour or two; it must be thoroughly cold if it is to be iced.