Preheat oven to 350°F (175°C). Grease and line a 9-inch (23 cm) round cake tin.
In a bowl, mix together the self-raising flour, caster sugar, soft margarine, eggs, and cocoa powder until all ingredients are well combined.
Melt the dark chocolate in a heatproof bowl over simmering water. Once melted add to the wet ingredients and mix together until fully incorporated.
Gently fold in the milk until a smooth batter is formed.
Pour the batter into the prepared cake tin and bake for 30-35 minutes or until an inserted skewer comes out clean.
Once baked, remove from oven and cool on a wire rack before removing from the tin and decorating as desired.