Step 1 Heat the oven to 375 degrees. Line the bottom of a 9-inch round cake pan with parchment paper, then grease with 1 teaspoon butter.
Step 2 In a small saucepan, combine the chocolate and the remaining one-half cup butter over low heat and melt, stirring often until smooth. Stir in the vanilla, remove from the heat and cover to keep warm.
Step 3In a medium bowl, using an electric mixer, beat one-half cup of the granulated sugar with the egg yolks until pale in color. In a large bowl, using clean beaters, beat the egg whites until soft peaks form. Slowly add the remaining one-fourth cup granulated sugar to the egg whites, beat until stiff peaks form, about 30 seconds.
Step 4Using a plastic spatula, gently stir the egg yolk mixture into the egg whites just until combined. Then gently stir in the melted chocolate until the mixture is a uniform chocolate color.
Step 5Pour the batter into the prepared pan. Bake the cake until a fork or toothpick inserted in the center comes out dry, about 35 to 40 minutes. Let cool in the pan on a rack for 10 minutes, then invert onto a serving plate and let cool completely. Remove the parchment paper. Using a fine-mesh sieve, dust the top of the cake with the confectioners' sugar. Top each serving with a dollop of whipped cream.