In a double boiler melt the chocolate and 1/2 of the brown sugar.
Stir together the malted milk powder, cocoa and boiling water in a small bowl. Once the chocolate has melted add the malted milk mixture and whisk gently until smooth. Allow to cool to room temperature.
In a mixing bowl cream together the butter and remaining 1/2 of the brown sugar until light and fluffy, about 2-3 minutes. Add the salt and vanilla. Turn the speed down and add the cooled chocolate mixture to the butter. Scrape down the bowl and continue mixing. Slowly add the confectioner's sugar and mix until smooth.
Cut the cake into 2 or 3 layers. Fill the layers with the malted buttercream and then cover the outside with more icing.