Take whatever you need out of the fridge so that all the ingredients can come to room temperature (though it's not so crucial here, since you're heating the milk and butter and whisking the eggs).Preheat the oven to 170°C/150°C/325°F. Butter and line two 20cm /8-inch loose-bottomed sandwich cake tins with baking parchment.Whisk together the sugars and eggs while you weigh out the ingredients; I use a freestanding mixer here which explains why I can do both things at once. Heat the milk, butter and Horlicks powder in a saucepan until the butter melts, and it is hot but not boiling. When the sugars and eggs are light and frothy, beat in the hot Horlicks mixture and then fold in the flour, cocoa, baking powder and bicarbonate of soda. Divide the cake batter evenly between the two tins and bake in the oven for 25 minutes, by which time the cakes should have risen and will spring back when pressed gently. Let them cool on a rack for about 5-10 minutes and then turn out of their tins. Once the cakes are cold, you can get on with the icing. I use processor just because it makes life easier: you don't need to sieve the icing sugar. So: put the icing sugar, cocoa and Horlicks in the processor and blitz to remove all lumps. Add the butter and process again. Stop, scrape down, and start again, pouring the boiling water down the funnel with the motor running until you have a smooth buttercream. Sandwich the cold sponges with half of the buttercream, and then ice the top with what is left, creating a swirly top rather than a smooth surface. Stud the outside edge with a ring of Maltesers or use them to decorate the top in whichever way pleases you.