Preheat the oven to 180°C/Fan 160°C/Gas 4. Grease 2x20cm round sandwich tins with butter and line the bases with baking paper.
Measure the malted chocolate drink powder and cocoa powder into a large bowl, pour over 2 tablespoons of water and mix to a paste. Add the remaining cake ingredients and beat until smooth.
Divide evenly between the prepared tins and bake in the oven for 20-25 minutes. Set aside in the tins to cool for 5 minutes, then turn out on to a wire rack to cool completely.
To make the icing, measure the malted chocolate drink powder into a bowl, add the hot milk and mix until smooth. Add the butter, icing sugar and melted chocolate and mix again until smooth, then add the boiling water to give a gloss to the icing.
Place one cake on a plate and spread over half the icing. Sandwich with the other cake and spread (or pipe) the remaining icing on top, using the tip of a rounded palette knife to create a swirled effect from the centre to the edge of the cake.
Arrange the Maltesers over the top and dust with icing sugar before serving.