Beat the butter and sugar together in a mixing bowl until pale and creamy. Beat in the eggs, a little at a time, until incorporated. Sift in the flour and salt and fold into the creamed mixture with the milk and vanilla. Alternatively, beat all the cake ingredients together in a food processor until smooth.
Transfer the mixture to an oiled and base-lined 18 x 25 cm (7 x 10 inch) cake tin. Spread the surface level with a palette knife and bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 25–30 minutes until risen and firm to the touch. Leave the cake to cool in the tin for 5 minutes then loosen the edges, turn out on to a wire rack and peel off the lining paper. Leave out overnight.
Make the icing. Sift the icing sugar and cocoa powder into a bowl, make a well in the centre and beat in the boiling water to make a smooth chocolate icing with a pouring consistency.
Cut the cooled cake into 24 pieces. Use 2 forks to dip each cake into the icing and then immediately coat with the coconut all over. Leave to set on baking paper.