Preheat oven to 160°C (140°C fan-forced) and line 2 x 20cm-round cake tins with non-stick baking paper.
Melt the butter and dark chocolate in a saucepan over medium heat.
Allow this to cool to room temperature before adding the cocoa,
buttermilk, vanilla and eggs and whisk to combine.
Place the flour, baking powder and sugar in a seperate bowl. Add the
wet mixture in gradually and whisk until smooth, then whisk on high for
3 minutes or until fluffy. Gently fold through walnuts.
Divide this mixture between cake tins and bake for 45-50 min or
until a skewer inserted into the center comes out clean. Allow the cakes
to cool for 10 min before removing from tins and place on a wire rack
to cool completely.
For Frosting: Using an electric mixer, place the butter in a bowl
and mix for 3 min or until pale and creamy. Add the soft icing mixture,
cocoa and milk. Mix for a further 5 min, until mixture is fluffy and
smooth.
To Assemble: Using a long bread knife, trim the tops of the cakes
(if needed) so they are flat. Place one cake layer on a plate and spread
with half of the frosting. Place the second layer and top with the
remaining frosting. Decorate with fresh berries and enjoy!