Preheat the oven to 180°C, fan 160°C, gas 4. Line 2 x 18cm round cake tins with the cake tin liners - these will give the cake edges an attractive texture.
In a large mixing bowl, beat together the butter and caster sugar, until light and creamy. Add the eggs, a little at a time, beating well after each addition. Add a little flour if the mixture starts to curdle.
Add the flour, cocoa powder and milk and beat until smooth. Divide the mixture between the 2 tins and bake on the middle shelf of the oven for 25 minutes, until well risen. Remove from the oven, lift the cakes out onto a wire rack and leave to cool completely in the liners.
Meanwhile, make the icing. Using an electric hand mixer, beat the butter or margarine in a large bowl until fluffy. Add the icing sugar and cocoa powder and continue to beat until smooth and creamy. As the mixture becomes a little dry, add 3 tablespoons boiling water and mix well.
Sandwich the 2 cakes together with half the icing. Spread the remaining icing on top of the cake and create peaks, using the back of a spoon, before it sets, as this will help prevent the decorations from rolling off. Sprinkle with the sugar stars.