Coat 9" springform pan with 2 teaspoons butter, and dust with cocoa powder. (Do not tap out excess cocoa.)
In food processor, fitted with steel blade, combine blanched almonds with 2 tablespoons of sugar. Process until finely ground.
In top of double boiler, over barely simmering water, melt chocolate and the remaining 4 teaspoons butter, stirring occasionally, until smooth.
Remove from heat. Place chocolate mixture in large bowl. Add almond mixture, sour cream, sugar substitute, egg yolks, vanilla extract, almond extract, and 1/4 cup of remaining sugar. Stir until well-blended.
In large bowl, with electric mixer on high speed, beat egg whites and salt until frothy. Gradually add remaining 2 tablespoons sugar, beating until stiff, glossy peaks form.
Stir one-quarter of the beaten whites into the chocolate mixture. Gently fold in remaining whites until no white streaks remain. Place in prepared pan. Gently smooth top.
Bake 30 minutes, or until cake has risen, the top is dry, and a wooden pick inserted in the center comes out with a few moist crumbs.
Place pan on cooling rack and cool until warm. The cake will fall dramatically. Loosen edges of cake with knife and remove pan sides.
Place slivered almonds in shallow baking pan, toast in oven 2-3 minutes, until golden brown. Sprinkle on top of cake before serving.