Preheat oven to 180°C/160°C fan-forced. Grease a 6cm-deep, 20cm (base) round cake pan. Line base and side with baking paper.
Place chocolate and butter in a small saucepan over low heat. Cook, stirring, for 10 minutes or until melted. Set aside to cool.
Place flours, bicarbonate of soda, sugar and sifted cocoa in a large bowl. Stir to combine. Whisk eggs and buttermilk in a jug. Stir buttermilk, then chocolate, into flour mixture. Pour into prepared pan. Bake for 1 hour 20 minutes or until a skewer inserted into the centre comes out clean. Stand cake in pan for 15 minutes. Cool on a wire rack.
Meanwhile, make chocolate ganache: Place chocolate and cream in a saucepan over low heat. Cook, stirring, for 3 to 4 minutes or until smooth. Cover. Refrigerate for 30 minutes or until a spreadable consistency.
Split cake in half. Place base of cake on serving plate. Spread over half the ganache. Sandwich with cake top. Spread top and side of cake with remaining ganache. Top with chocolate curls, if using. Stand for 10 minutes or until ganache sets slightly. Serve.
