Heat oven to 325 degrees F. Coat an 8-1/2-inch springform pan with nonstick cooking spray. Wrap pans bottom and side with foil to prevent water bath from seeping into pan. Put kettle of water on to boil.
Beat eggs with hand mixer on high speed for 5 minutes or until eggs double in volume.
Combine chocolate and butter in a large microwave-safe bowl and heat on 50 percent power for 4 minutes or until chocolate has melted, stirring twice. Fold eggs into chocolate in 3 additions until fully mixed in.
Pour batter into prepared pan. Place cake pan in a large roasting pan in oven. Pour boiling water into roasting pan to come halfway up side of cake pan. Bake at 325 degrees F for 40 to 47 minutes or until wooden pick inserted in center comes out completely clean.
Remove cake to a wire rack and cool completely. Dust with confectioners sugar before serving, if desired.