Combine the water, salt, and sugar in a small saucepan over medium heat. Stir until completely dissolved and set aside.
Either in the top half of a double boiler or in a microwave oven, melt the bittersweet chocolate.
Pour the melted chocolate into the bowl of an electric mixer.
Cut the butter into pieces and beat the butter into the chocolate, 1 piece at a time.
Beat in the hot sugar-water.
Slowly beat in the eggs and the egg whites one at a time.
Pour the batter 3/4 to the top of lined cupcake tins. These don't rise (hence the kosher for Passover status) so you can go higher if you'd like.
Have pans larger than the cupcake tins ready. Put each cupcake tin in a larger pan and fill the pan with boiling water halfway up the sides of the cake pan. (You can get an idea of how I did this in the picture at the right. Mine could actually be filled a bit higher. I filled it more in my second batch.)
Bake cupcakes in the water bath at 300 degrees F (150 degrees C) for 30 minutes. The center will still look wet.
Do not take the cupcakes out of the tin! Wait until the tins are cool enough to handle with your bare hands, then put the entire cupcake tin in the fridge until it is cool. Then remove the cupcakes from the tin and frost. If you take the cupcakes out of the tin while they are still hot, they will lose their shape.