In a medium pot, warm the milk over a medium heat with the chocolate, malted milk powder and granulated sugar (stirring so that the sugar dissolves) until gently steaming.

Place the cornstarch into a small bowl and pour in some of the hot milk mixture. Stir together to get a slurry then pour this into the pot and return to the heat. Keep stirring over a medium-low heat until thickened then stir in the vanilla extract. Remove from the heat and leave to cool to room temperature before covering and chilling until completely cold. (this is the chocolate pudding)

Once the pudding has cooled, place the softened butter into a large bowl. Cream the butter together with the cocoa powder and salt until smooth and slightly fluffy. Gradually stir in spoonfuls of the cold pudding, beating well between additions to incorporate it fully, until you have a smooth, spreadable frosting (you may not need all of the pudding).