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.