Preheat oven to 350° F. Butter and flour a round 9-inch springform pan. Line the bottom with parchment paper. Set aside.
In a microwave safe bowl, melt together finely chopped chocolate and butter at half power for 60-90 seconds. Stop every 15 seconds to stir mixture. Continue to heat until mixture is smooth and melted. Make sure microwave is at half power, otherwise the chocolate will burn. Alternatively, melt chocolate and butter over a bain-marie.
In a large bowl, whisk together egg yolks and brown sugar. Beat until airy and light in color. Stir in vanilla and slightly cooled chocolate-butter mixture.
In another bowl, whisk together egg whites and salt. Beat until soft peaks. Gently fold egg whites into cake batter in three additions. Add sifted cornstarch and gently fold in until even distributed.
Pour batter into prepare cake mold. Bake for 15-20 minutes until top of cake springs back when touched and toothpick inserted in center of cake comes out clean. Let cool slightly in pan before unfolding. Let cool on wire rack. The cake will slightly shrink in diameter.
Carefully remove parchment bottom. Cut cake to fit inside an 8-inch ring mold. Line ring mold with plastic cake wrap. Gently place cake at the bottom of the mold. Set aside as you prepare chocolate mousse.