For the cake:

Preheat the oven to 350°F. In a mixing bowl, mix together the baking soda and sour cream with a wooden spoon and let sir for 5 minutes. Grease two 9-or 10-inch cake pans, or use nonstick pans.

Place the chocolate and butter in a medium bowl and pour the boiling water over them. Whisk in the vanilla and sugar, then whisk in the yolks one at a time. Add the sour cream mixture and blend. Combine the flour and baking powder and add to the chocolate mixture, mixing just until combined.

In the bowl of an electric mixer, using the whisk attachment, or using a hand-held electric mixer, heat the egg whites at high speed to soft peaks. Fold them into the chocolate mixture.

Pour the batter into the prepared pans and bake until lightly springy, about 30 minutes. Let cool 10 minutes in the pans, then invert onto racks and cool to room temperature.

For the ganache:

In a small bowl set over a pot of simmering water, heat the chocolate and cream together, stirring, until smooth and combined. Keep it warm over the water or on a warm pan of your stove top until ready to use.

To finish the cake:

When the cakes are well cooled, pour the ganache over the top and let it run down the sides. Smooth it with a spatula, filling in any gaps. Sprinkle with the optional almonds.

For a 2-layer cake, cool one third of the ganache in a small bowl at room temperature, set in a larger bowl of ice cubes, or in the refrigerator, until somewhat stiff. Keep the remaining ganache warm in a bowl set over water that has been brought to a simmer and removed from the heat, cut a thin slice off the top of each layer with a serrated knife, leaving a flat surface. Spread the stiffened ganache over one layer and cover with the second. Pour the warm ganeche over the top.