First, be sure to leave your ingredients out for a couple of hours to come to room temperature. Buttercream is near on impossible to make using butter straight from the fridge.
Preheat the oven to 160 C / Gas 3. Butter and line your spherical cake tins with baking parchment. I used the technique shown in the photo to make sure I could easily remove my cakes from the tins.
Heat the milk, butter and Horlicks® powder in a small saucepan until warmed through and the butter has melted.
In a large mixing bowl, beat the brown and caster sugar together till light and foamy. Gradually add the milk mixture into the egg mixture, beating constantly.
Sift together the flour, cocoa, baking powder and bicarb. Add to the wet mixture and stir well till thoroughly combined. Divide the cake mixture evenly between the two tins.
Bake in the oven for 35 to 40 minutes, or until they've risen and spring back when pressed gently. Let them cool on a rack for 10 minutes, then turn them out of their tins.
Once your cakes are cool, you'll need to level them up so they fit together nicely. Then carefully using a bread knife divide the sponge into four layers.
You can now get on with the icing. I use a processor just because it makes life easier: you don't need to sieve the ingredients. You may want to divide the quantities for the icing into two batches as it makes a fair bit which probably will not all fit into your processor. So, place the icing sugar, cocoa and Horlicks® in the processor and blitz to remove all lumps. Then 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.
It's now time for the assembly. On your plate or whatever you want to present your cake on, put a small amount of buttercream in the centre to secure the cake. Get the bottom layer of your cake and place it onto the butter cream and, using a palette knife (or a butter knife), spread on a 5mm of buttercream. Do the same with the rest of your layers until you're left with a ball.
Now cover your cake with 5mm of the remaining buttercream. To cover the undersides of the cake with your Maltesers® you may want to chop a few in half. Then you can just go for it and cover your cake in Maltesers®. Make sure to press them in firmly but be quick as you don't want them to melt.