Microwave for 30 seconds at a time. Stir and reheat in microwave until completely melted. Be careful not to burn chocolate.
Set aside to cool.
Cake Batter
Beat egg whites to a stiff peaks.
5 eggs
Mix sugar and a pinch of salt into chocolate mixture.
2 tablespoons sugar, 1 pinch salt
Mix egg yolk to chocolate mixture.
Gently fold in egg whites.
Pour cake mixture into prepared cake pan.
Place in oven in center of prepared roasting pan with water.
Bake at 325 degrees for 30 -35 minutes. Gently touch top of cake with your finger. If the cake is jiggly, cook for an additional 3-5 minutes.
Fully cool cake in cake pan on a baking rack.
Run a knife around sides of cake. Place hand on top of cake and invert cake pan. Remove parchment and place cake on a cake plate.
Dust with powdered sugar just before serving.
Notes
When adding beaten egg whites, fold in carefully so you don't deflate the egg whites.
Be certain all egg whites are fully incorporated into chocolate mixture. Just keep folding until everything is mixed well.
Be certain to use parchment paper in your cake pan. This will ensure your cake can easily be removed from the pan. I usually spray a bit of nonstick spray in the bottom of my cake pan and then add the parchment paper. Then I spray the parchment paper and the sides of the pan.
To make a perfect size parchment paper, lay the cake pan on top of the paper and trace the bottom of the pan with a pencil onto the paper. Then cut out a perfectly formed circle.