Place rack in the lower third of oven; preheat oven to 375 °F. Butter and flour a 9-inch spring form pan.
In double boiler top over simmering water, melt chocolate with butter. Keep chocolate warm over a pan of warm water.
In a large bowl of mixer, beat egg yolks at high speed. Gradually add 3/4 cup of the granulated sugar, beating until mixture is pale yellow and thick, 4-6 minutes. Add chocolate mixture to yolk mixture; beat until completely smooth.
Add creme de cacao and vanilla; beat until blended. In a medium bowl of mixer, beat egg whites at high speed until soft peaks form. Gradually add remaining 1/4 cup sugar, beating until stiff but not dry peaks form.
Fold egg whites gently but thoroughly into chocolate mixture. Pour evenly into reserved pan; smooth top.
Bake 15 minutes. Reduce oven temperature to 350 °F and bake another 15 minutes. Reduce oven temperature to 250 °F and bake 30 minutes longer. (Total baking time is 1 hour.) Turn oven off; leave cake inside with oven door propped open for 30 minutes. Remove cake from oven and cover with damp paper towel; let stand for 5 minutes.
Remove towel and let cake cool completely. Dome of cake will crack and collapse; this is normal. Press top down lightly to smoothen. Remove spring form pan and transfer cake to serving platter.
In chilled bowl of mixer, whip cream on high speed until soft peaks form. Gradually add 1 1/2 tablespoons of the powdered sugar, beating until stiff peaks form.
Just before serving, dust top of cake with remaining powdered sugar. Serve cake at room temperature with whipped cream.