Preheat oven to 325 degrees.
In a small saucepan, melt chocolate and butter over very low heat.
In a large bowl, beat egg yolks and 3/4 cup sugar until very light and fluffy, about 5 minutes. Gradually beat in warm chocolate mixture and vanilla.
In another large bowl, beat egg whites with cream of tartar until soft peaks form. Add remaining 1/4 cup sugar 1 tablespoon at a time. Continue beating until stiff.
Fold egg whites carefully into chocolate mixture. Pour 3/4 of the batter into an ungreased 9×3 springform pan. Cover remaining batter and refrigerate. Bake cake 35 minutes.
Prepare Whipped Cream Frosting and set aside.
Remove cake from oven and cool. Cake will drop as it cools. Remove outside ring of springform pan. Stir refrigerated batter to soften slightly. Spread on top of cake. Refrigerate until firm.
Spread Whipped Cream Frosting over tops and sides. Garnish with Chocolate Leaves, if desired. Refrigerate several hours or overnight.
Whipped Cream Frosting
In a small bowl, beat whipping cream until soft peaks form. Add powdered sugar and vanilla. Beat until stiff.