To make the Genoise:

Preheat oven to 350°F. Wrap outside of 9-1/2-inch-wide by 3-inch-high heart-shaped springform pan with foil. Butter and flour pan. Mix flour and salt in small bowl. Stir butter and vanilla in glass measuring cup.

Whisk eggs and 3/4 cup sugar in large stainless steel bowl to blend. Set bowl over saucepan of simmering water (do not allow bowl to touch water). Whisk constantly until mixture is very warm to touch, about 3 minutes. Remove from over water. Using electric mixer, beat mixture at high speed until it triples in volume and is thick enough to fall in heavy ribbons that will sit on the top of the batter for a moment when beaters are lifted, about 10 minutes. Sift flour over batter in 4 additions, gently folding just to combine after each addition. Drizzle butter mixture over while gently folding to combine (do not overfold or batter will deflate). Transfer batter to prepared pan.

Bake génoise until golden and tester inserted into center comes out clean, about 25 minutes. Cool in pan on rack; cake will fall slightly. (Can be made 1 day ahead. Cover tightly; store at room temperature.)

To make Soaking Syrup:

Stir 1/2 cup sugar and 1/2 cup water in heavy small saucepan over low heat until sugar dissolves. Increase heat and bring to boil. Remove from heat; mix in lemon juice. Cool. (Can be prepared 1 day ahead. Cover and let stand at room temperature.)

To make Mousse:

Using rubber spatula, push 1/2 cup raspberries through sieve into small bowl. Set berry puree aside; discard seeds. Pour 1/4 cup Champagne into another small bowl. Sprinkle gelatin over Champagne; let stand until gelatin softens, about 10 minutes.

Whisk 6 tablespoons sugar, yolks, lemon juice and 1/4 cup Champagne in medium-size stainless steel bowl to blend. Set bowl over saucepan of simmering water (do not allow bowl to touch water). Whisk constantly until mixture is thick and fluffy and candy thermometer registers 160°F, about 2 minutes. Remove bowl from over water. Add chocolate and gelatin mixture. Set bowl over simmering water again; whisk gently until chocolate melts and gelatin dissolves. Remove bowl from over water. Using rubber spatula, gradually fold in 1/2 cup Champagne. Chill mousse base until cold and beginning to thicken but not set, about 20 minutes.

Beat cream with 2 tablespoons sugar in medium bowl to stiff peaks. Fold in raspberry puree. Fold raspberry cream into mousse base. Fold in more berries. Let stand at room temperature while beginning cake assembly or up to 30 minutes,

To Assemble and Decorate:

Cut around pan sides to loosen cake. Release pan sides. Using large metal spatula, loosen cake from pan bottom. Transfer cake to work surface. Clean cake pan.

Cut cake in half crosswise, making 2 equal layers. Return bottom layer, cut side up, to cake pan bottom. Brush layer with 5 tablespoons soaking syrup; then spread with 1/4 cup jam. Re-attach sides of pan. Spoon mousse in even layer over jam. Place second cake layer, cut side down, on mousse. Brush top of cake with 5 tablespoons syrup; then spread with 1/4 cup jam. Cover pan; chill until mousse sets, about 4 hours or up to 1 day.

Using sharp knife, cut around pan sides to loosen cake. Release pan sides. Run icing spatula around cake to remove excess mousse (mousse should be flush with sides of cake). Slide large metal spatula under cake on all sides to loosen from pan bottom. Using tart pan bottom as aid, transfer cake to platter.

Arrange raspberries on top of cake, leaving 3/4-inch border at edge. Beat cream with sugar in large bowl until stiff peaks form. Spread about 2/3 of cream around sides of cake. Transfer remaining cream to pastry bag fitted with medium star tip. Pipe border around edge of cake. Cover cake with cake dome; chill up to 8 hours.