1. Heat oven to 300 degrees. 2. Butter a 9-inch springform pan and line the side wall with a strip of parchment paper. 3. In the bowl of a double-boiler, combine butter and chopped chocolate. Add water to bottom of pan and place bowl over pan. Melt chocolate over medium heat until smooth. Remove bowl from pan and cool slightly. (I cheated and did mine in the microwave - I don't have a double-boiler.) 4. Beat egg whites with an electric mixture until soft peaks form. Slowly add sugar and beat until glossy and stiff. 5. Whisk egg yolks into chocolate mixture. Whisk in a dollop of egg whites to lighten. Gently fold in the rest of the egg whites with a rubber spatula. 6. Pour batter into prepared pan and bake until cake is set (center may be jiggly), about 40-45 minutes. 7. Cool cake for 20 minutes and remove sides of pan. Continue to cool. Dust with confectioners sugar. This taste great with a little dollop of ice cream or cool whip as well!
This flourless cake has a light, mousse-like texture. Best served warm.