Dissolve the malted milk powder in the hot milk and leave aside to cool completely
Put 325g/ 11 ½ oz sugar and the water into a medium saucepan then place on medium-low heat and cook until the sugar dissolves. Keep on a low heat while you prepare the egg whites, then, when the meringue reaches the soft peak stage, raise the heat and cook until the syrup reaches a temperature of 115C (240F)
Beat the egg whites and cream of tartar in the bowl of a stand mixer. Add 75g/ 3 oz of sugar spoonful by spoonful and then keep beating until soft peaks form. When the temperature of the sugar syrup reaches 110C (230F) beat the meringue on high until the stiff peak stage
Once the syrup reaches 115C (240F) drizzle it slowly into the stiff egg whites whilst the mixer is still running. Take care to pour the syrup into the side of the bowl directly onto the meringue and not onto the metal whisk (to avoid the risk of the incredibly hot syrup splattering you)
Continue to whip the meringue until it is lukewarm
Switch to a paddle attachment. Add room temperature butter into the running mixer one tablespoon piece at a time, then add the vanilla extract and the cooled malted milk. Beat until everything is combined and the mixture has reached a silky consistency
Use approximately ⅔ to sandwich and cover the cake and flavour the remaining ⅓ with cocoa powder (sieved) - I added 1 ½ teaspoons, removed half of the buttercream and added another 1 ½ tsp to the remaining. This added a little colour variation and flavour intensity to the piped buttercream