Grease and line one medium roasting tin - ideally with straight sides.
Put the caster sugar, eggs, oil, milk, and golden syrup in a bowl. Whisk together.
Sift in the self-raising flour, malt powder, cocoa powder and bicarbonate of soda. Whisk again to a smooth batter.
Pour into the prepared tin and bake for approximately 1 hour until firm and springy in the centre.
Cool for 30 minutes in the tin on a wire rack, then turn out to cool completely.
Make the frosting.
Put the butter in a large bowl and whisk until pale and creamy.
Sift in the icing sugar, cocoa and malt milk drinking powder, then add the milk and vanilla. Beat until very pale and fluffy.
Spread all over the top of the cake, using a palette knife or silicone spatula to spread it evenly to the edges of the cake.
Chop the Maltesers quite roughly so that you have some small pieces, some halves and some whole Maltesers. Scatter them over the frosting - you may need to pat them down slightly just to ensure they stick.