Pre-heat the oven to 350° F (175° C). Liberally butter a 9" spring-form pan.
In a medium heatproof mixing bowl, combine chocolate and butter. Microwave on high for 1 minute; stir every 30 seconds. Cool the mixture to lukewarm, about 2-3 minutes; reserve.
In an electric mixer with a wire whip attachment at medium speed, whip egg yolks and 6 tablespoons of Splenda until mixture turns pale yellow (about 3-4 minutes).
Fold in the chocolate mixture until just combined. Continue mixing and add vanilla extract until just combined. Transfer mixture to a medium bowl.
Thoroughly clean mixing bowl and whip attachment. In cleaned bowl, whip egg whites until they hold soft peaks, about 2-3 minutes. Slowly add in remaining sugar and whip until mixture holds medium-firm peaks, about 1-2 minutes more. In thirds, gently fold whites into the chocolate mixture until just combined. Pour batter into prepared pan.
Bake until top of cake has crusted and cracks open, about 50 minutes. Cool on wire rack for 10 minutes, cake will fall. Invert onto plate, keeping pan sides around the cake. Cool for 30 minutes more.
Invert cake to right side up; run a small knife around the edge of the pan to release the cake. Remove pan sides; let cake cool completely.
Wrap the cake in plastic wrap; let stand at room temperature overnight. To serve, slice and add a dollop of whipped cream or some berries.
There are 205 calories in 1 serving of Flourless Chocolate Cake II.