Heat chocolate in small saucepan over very low heat, until just melted and smooth, stirring once. Set aside to cool completely.
Adjust oven rack to top of lower third of oven. Heat oven to 350°F. Grease 10 x 2-inch springform pan. Line bottom with waxed paper; grease paper; flour paper and pan.
Beat together eggs, yolks, 3/4 cup buttermilk and vanilla in bowl until blended.
Combine flour, brown sugar, baking powder and salt in large bowl. Beat on low speed 30 seconds.
Add cooled chocolate, remaining buttermilk, butter and oil to flour mixture. Beat on low speed just until dry ingredients are moistened. Increase speed to medium; beat for 1-1/2 minutes or until blended.
Add one-third egg mixture to batter. Beat on medium speed 20 seconds. Repeat until all the egg mixture is used. Pour into the prepared pan.
Bake in 350°F oven 65 to 75 minutes or until wooden pick inserted in center comes out clean. If top begins to brown too quickly, cover loosely with foil for last 20 minutes of baking. Cool cake in pan on rack 15 minutes. Loosen sides of pan; remove. Using long thin metal spatula, loosen cake from bottom of pan. Using 2 wide pancake spatulas, transfer cake to rack to cool. Remove waxed paper.
Meanwhile, prepare frosting.
Trim cake top level if necessary. Place cake, bottom side up, on platter. Spread refrigerated ganache over sides and top of cake, smoothing with long metal spatula. Chill 1 hour or until ready to serve. If chilling longer than 1 hour, let stand at room temperature 1 hour before serving. Garnish top with berries if desired.
Bring heavy cream to boiling in small saucepan over medium-high heat. Remove from heat. Stir in squares of semisweet chocolate, chopped, until smooth. Stir in vanilla. Pour into bowl; refrigerate until good spreading consistency, 2 hours.