Preheat the oven to 180 degrees C. Grease and flour a 25cm ring tin.
In a large bowl blend together the butter, white sugar, egg yolk and vanilla until smooth. Beat in eggs one at a time using an electric mixer. Combine flour, cocoa, bicarbonate of soda and salt then stir into the egg mixture alternately with the sour cream and water. Pour batter into the prepared cake tin.
Coconut Macaroon Filling: In a separate bowl with clean beaters whisk the egg white until soft peaks form. Gradually sprinkle in white sugar while continuing to whip to firm peaks. Fold in the coconut, flour and vanilla by hand using a spatula or wooden spoon. Drop this mixture by teaspoonfuls over the chocolate batter in the cake tin. Be careful not to let the filling touch the sides of the tin.
Bake for 55 to 65 minutes in the preheated oven or until a knife inserted into the cake comes out clean. Cool for 15 minutes in the tin then invert onto a wire rack to allow the cake to cool completely. Remove cake from tin and drizzle with vanilla glaze.
Vanilla Glaze: Mix together the icing sugar, butter and milk in a small bowl gradually adding milk until the mixture is thick but pourable. Drizzle over cooled cake.