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 separate 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 centre comes out clean. Allow the cakes to cool for 10 minutes 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 minutes or until pale and creamy. Add the soft icing mixture, cocoa and milk. Mix for a further 5 minutes, 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!
200g unsalted butter, chopped into cubes
200g NESTLÉ BAKERS’ CHOICE Dark Chocolate, roughly chopped