Beat the butter and sugar until creamy. Separate the eggs. Gradually stir the yolks into the butter mixture. Let the chocolate melt over a hot water bath and stir into the butter mixture. Mix the almonds, salt, baking soda and flour. Fold into the butter mixture.
Beat the egg whites until stiff and fold into the mixture.
Pour the batter into a greased, rectangular pan. Bake for 20-25 minutes at 180°C (approximately 350°F). Remove and cool on a wire rack.
Sprinkle with powdered sugar and serve.