Preheat oven to 180C (160C fan/ Gas Mark 4). Grease and line two 20cm (8 inch) round cake tins.
In a large bowl, cream together the softened butter and granulated sugar until light and fluffy.
Beat in the eggs one at a time, then stir in the vanilla extract.
In a separate bowl, whisk together the self-raising flour and cocoa powder.
Gradually add the dry ingredients to the wet ingredients, alternating with the milk, beginning and ending with the dry ingredients. Mix until just combined.
Stir in the melted dark chocolate until evenly distributed.
Pour the batter evenly into the prepared cake tins.
Bake for 30-35 minutes, or until a skewer inserted into the center comes out clean.
Let the cakes cool in the tins for 10 minutes before transferring them to a wire rack to cool completely.
For the Fudge Icing: In a large bowl, beat together the butter, icing sugar, and cocoa powder until smooth. Gradually add the milk, mixing until combined. Stir in the melted dark chocolate.
Once the cakes are completely cool, spread a layer of fudge icing over one cake layer, top with the second cake layer, and frost the entire cake with the remaining icing.
Slice and serve.
Optional Tips: For a richer flavour, substitute strong brewed coffee for the milk in the cake batter. Chopped nuts or chocolate chips can be added to the batter for added texture. If self-raising flour is not available, use plain flour and add 2 teaspoons of baking powder.