Preheat oven to 325°. Butter a 10" springform pan; dust with sugar.

For cake: Melt chocolate and butter in heavy large saucepan over low heat, stirring constantly. Cool to lukewarm. Whisk in sugar. Whisk in eggs one at a time, blending well after each addition. Mix in vanilla and salt, then flour.

Pour into prepared pan. Bake at 325° until cake just rises in center, 30-35 minutes. Cool completely in pan on rack. (My cake fell in the middle, that's alright.) Cover and chill while making the mousse.

For mousse: Melt butter in medium metal bowl set over saucepan of simmering water. Don't let the bowl touch the water. (Use a double boiler if you have one.)

In a small bowl, whisk egg yolks, 1/4 cup cream and vanilla. Gradually whisk yolk mixture into bowl with the melted butter. Whisk constantly over simmering water until thermometer reaches 150°, about 6 minutes. Remove from stove; add chocolate and stir till melted. Set aside.

Beat egg whites with sugar and cornstarch in large bowl to medium-stiff peaks. Whisk 1/4 of beaten egg white mixture into warm chocolate mixture to lighten. Fold in remaining egg white mixture. Spread over cake. Chill torte until mousse is set, at least 6 hours and up to 1 day.

Topping: Run sharp knife around edge of pan to loosen torte. Release pan sides. Transfer torte to platter. Beat cream and powdered sugar in medium bowl until peaks form. Spread over torte. Garnish as desired.