Preheat oven to 250 degrees. Line 2 baking sheets with parchment paper and trace three 8-inch circles onto paper.
In a large bowl with an electric mixer beat whites with cream of tartar and a pinch salt until they just hold soft peaks. Add 1 cup sugar gradually, beating, and beat meringue until it holds stiff peaks. Transfer meringue to a pastry bag fitted with a 3/4-inch plain tip and pipe onto circles, filling them in. Smooth meringues with a long metal spatula or knife. (Alternatively, if pastry bag isn't used, circles may be made by spreading meringues freehand with spatula.)
Bake meringues 30 minutes and switch positions of baking sheets in oven. Bake meringues 30 minutes more, or until pale golden and crisp. (If the weather is humid, cooking time may be longer.) Cool meringues completely and peel off paper. Meringues may be made 1 day ahead and kept in turned-off oven.
In a saucepan bring 1/4 cup heavy cream just to a boil and remove from heat. Add chocolate and stir mixture until chocolate is melted completely. Divide ganache between 2 meringue layers and gently spread evenly, leaving 1/2-inch borders. Chill ganache until hardened, about 10 minutes.
In a small saucepan sprinkle gelatin over water and soften 1 minute. Heat mixture over low heat, stirring until gelatin is dissolved, and keep warm. In a bowl with an electric mixer beat remaining 2 1/2 cups heavy cream with remaining 1/2 cup sugar until it just holds stiff peaks and beat in sour cream and vanilla. Add gelatin mixture in a stream, beating, and beat until mixture holds stiff peaks.
Arrange 1 ganache-topped meringue on a plate and spread it with about 1 1/2 cups cream mixture, mounding it at the edge. Arrange 1 pint raspberries over cream mixture and top with second ganache-topped meringue. Repeat procedure with about 1 1/2 cups more cream mixture and another pint raspberries and top with third meringue. Transfer about 1 cup more cream mixture to pastry bag fitted with a star tip and reserve for decoration. Frost top and side of cake with remaining cream mixture and decorate with reserved mixture. Chill cake at least 2 hours and up to 8 hours. Remove wax paper carefully and scatter remaining pint raspberries on and around cake. Cut cake into 12 wedges with serrated or electric knife.
Courtesy of Gourmet Magazine