Line an 8 inch cake pan with parchment paper on the bottom. Spray sides and paper with nonstick spray.
Place chocolate and butter in a large microwave safe bowl.
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.
Beat egg whites to a stiff peaks.
Mix sugar into chocolate mixture.
Using a whisk, whisk in egg yolk to chocolate mixture.
Gently fold in egg whites.
Pour cake mixture into prepared cake pan.
Place cake pan into a large roasting pan. Place in oven.
Fill roasting pan with hot tap water. Water level should rise half way up cake pan level. Be careful not to let water drip into batter.
Bake at 325 degrees for exactly 30 minutes. Cake will be very moist.
Cool in pan on rack for 30 minutes.
Run a knife around sides of cake and invert onto a cake plate. Remove parchment.
Dust with powdered sugar just before serving.