Preheat the oven to 170C (340F). Line a 9x13inch rectangular cake pan with baking paper.
Put the flour, sugar, cocoa powder, malted milk powder, baking powder, bicarbonate of soda and stir together to combine.
Add the butter, eggs, milk and vanilla.
Start beating on the lowest speed setting until the ingredients start to come together.
Increase the speed gradually, until the batter is completely smooth. Stop the mixer and scrape the sides and bottom of the bowl to make sure everything is incorporated.
Transfer the batter into the prepared cake tin and bake for 35 -40 minutes.
Check the cake is done by inserting a skewer in the centre - it should come out clean, the cake should be well risen golden and springy to the touch. Cool the cake on a wire rack before frosting.
Chocolate Frosting
Put the chocolate in a bowl and melt in the microwave, in short 30 second bursts, until it melts. Alternatively, set the bowl over a pot of simmering water and allow if to melt gradually using this double boiler method. Leave the chocolate to cool slightly.
Put the icing sugar, cocoa powder and malted milk powder in a mixing bowl (or the bowl of your stand mixer) and mix together to combine.
Add the softened butter and start mixing on low speed, gradually adding the milk until the frosting is well combined.
Add the melted and cooled chocolate and beat together until the frosting holds soft peaks.
Assemble the Cake
Use a palette knife to spread the frosting over the cake, swirling the surface.
Add the Maltesers (I halved some but best left whole). Slice the cake using a serrated knife, wiping the knife clean between each slice.
Notes
This cake will keep for 2-3 days at room temperature but it is best eaten when fresh.