Position rack in center of oven and preheat to 350 F.
Butter 10-inch-diameter springform pan with 2-inch-high sides.
Melt both chocolates with coffee and butter in heavy medium saucepan
over medium-low heat, stirring until smooth.
Pour into large bowl. Cool.
Mix in vanilla extract.
Using electric mixer, beat yolks with both sugars in medium bowl
until slowly dissolving ribbon forms when beaters are lifted.
Fold yolk mixture into chocolate mixture. Fold in flour.
Using electric mixer fitted with clean, dry beaters, beat whites
in another large bowl until almost stiff. Fold 1/4 of whites into
chocolate mixture to lighten. Gently fold in remaining whites.
Gently pour batter into prepared pan.
Bake until toothpick inserted in center comes out with moist crumbs
still attached, about 45 minutes.
Transfer to rack and cool (cake will fall).
Can be prepared 1 day ahead. Cover cake in pan with plastic wrap
and let stand at room temperature.)
Run small knife around cake pan sides to loosen if necessary. Release
pan sides of cake. Invert cake onto plate.
Frost with Dark Chocolate Glaze.
If you prefer, just place a stencil or doily atop cake. Sift powdered
sugar over. Remove stencil from cake and serve.