Preheat the oven to 180 C / Gas 4. Grease two 20cm cake tins and line with baking parchment.
Sift flour, salt, cocoa powder and sugar into a bowl and make a well in the centre. Dissolve the bicarbonate of soda in a tablespoon of the milk and pour into the flour with the remaining milk, oil, syrup and vanilla extract and beat well to make a smooth batter. Divide the the mixture equally between the two prepared cake tins.
Bake in the preheated oven for 40 minutes or until the cake springs back when lightly pressed with the finger tips. Remove from the oven and allow to cool completely on a wire rack.
For the chocolate icing:
Place the cocoa in a bowl with the water and butter; beat until smooth and the butter has melted. Beat in the icing sugar to make a thick icing.
Place the first cake on a serving plate then spread the chocolate icing on top. Place the second cake on top and dust with icing sugar to serve, if desired.