- Portion size 12 servings
Grease sides of three 9-inch (1.5 L) round cake pans; set 2 aside. Using remaining cake pan as template, trace three 9-inch (23 cm) circles on parchment or waxed paper; cut out. Draw 6-inch (15 cm) circle, centred, on each cutout; draw 3-inch (8 cm) circle, centred, on each cutout. Line bottom of each pan with 1 paper circle, pencil side down.
In large heatproof bowl set over hot (not boiling) water, melt chocolate. Stir in cocoa powder. Let cool to room temperature. In separate bowl, combine flour, baking powder and baking soda. Set aside.
In large bowl, beat butter with sugar until light and fluffy. Beat in eggs, 1 at a time, beating well after each addition. Beat in vanilla. Beat in flour mixture alternately with buttermilk, making 3 additions of dry ingredients and 2 of buttermilk. Stir 3 cups (750 mL) of the batter into melted chocolate.
Spoon white and chocolate batters into separate large piping bags. (Or spoon into 2 large resealable plastic bags and cut 1-inch/2.5 cm hole in 1 corner of each.) Pipe white batter to fill outer ring of each of 2 of the pans. Pipe chocolate batter over middle ring, then white batter over inner ring. In third pan, pipe chocolate batter over outer ring, white over middle ring and chocolate over inner ring. Repeat with any remaining batter, piping over rings to make even layers, if necessary.
Bake in centre of 35°0F (180°C) oven until cake tester inserted in centre comes out clean, 25 to 30 minutes. Let cool in pans on rack for 10 minutes. Turn out onto rack; let cool completely. (Make-ahead: Wrap individually in plastic wrap and store at room temperature for up to 1 day or freeze in airtight container for up to 2 weeks.)
Place chocolate in bowl. In saucepan, heat whipping cream over medium heat just until steaming. Pour over chocolate, whisking just until melted. Stir in sour cream until smooth. Refrigerate until thickened but still spreadable, about 30 minutes. (Make-ahead: Cover and refrigerate for up to 1 week or freeze for up to 2 weeks; let thaw in refrigerator for 12 hours. Let stand at room temperature for about 1 hour or until spreadable.)
Whisk just until colour lightens.
Using serrated knife, trim tops of cake layers to make level, if necessary. Starting with 1 of the cake layers with white outer ring, spread with 1/2 cup (125 mL) of the ganache. Top with cake layer with chocolate outer ring; spread with another 1/2 cup (125 mL) of the ganache. Top with remaining cake layer. Cover top and side with remaining ganache. (Make-ahead: Refrigerate in airtight container for up to 1 day. Let come to room temperature before serving.)