PREPARATION:

1. Preheat oven to 350°F; set oven rack to middle position. Spray 8-inch round cake pan with nonstick cooking spray.
2. Beat cream with 2 tablespoons sugar substitute in large bowl at high speed of electric mixer until soft peaks form; set aside.
3. Place unsweetened and semisweet chocolate in large microwavable bowl; microwave on HIGH 2 to 3 minutes or until chocolate is melted, stirring after 1 minute and at 30-second intervals after the first minute.
4. Beat eggs and remaining 1 cup sugar substitute in large bowl at high speed of electric mixer about 7 minutes or until pale and thick. Add melted chocolate, coffee and salt to egg mixture; beat until well blended.
5. Fold whipped cream and 1/4 cup walnuts into egg mixture. Spread in prepared pan; sprinkle with remaining 1/4 cup walnuts. Place pan in large roasting pan; add enough hot water to roasting pan to reach halfway up side of pan. Bake 30 to 35 minutes or until set but still soft in center.
6. To unmold, loosen edge of cake with knife; place serving plate upside down over pan and invert. Serve warm garnished with whipped cream, if desired.