Preheat oven to 275 degrees, with rack in center. Butter a 9-inch spring form pan. Place butter and chocolate in large bowl. Microwave in 30-second increments, stirring each time, until melted; cool slightly. Whisk in yolks.
In another bowl, beat egg whites to soft peaks. Gradually add granulated sugar; beat until stiff and glossy. Whisk 1/4 of whites into chocolate mixture; gently fold mixture into remaining whites.
Pour into prepared pan; smooth top. Bake until cake pulls away from sides of pan and is just set in center, 45 to 50 minutes. Cool completely on a wire rack. Serve dusted with confectioners' sugar.
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