Pre-heat the oven to 175ºC (350ºF).
Line an 8x8” pan with parchment or aluminium foil.
Melt the butter and set to one side until it cools a little. Then, beat in the sugar followed by the egg. Lastly, add the vanilla extract.
Sift the flour and baking powder together and then stir into the wet ingredients, just until combined. Finally, stir the Maltesers through.
Pour into the prepared pan and bake for around 18-20 minutes. Remove from the oven and allow to cool completely.
For the frosting: sift the icing sugar and cocoa together and add to the cream cheese. Beat until nice and smooth. Spread over and then pop a few Maltesers on top, if you like (see note 3). Slice and enjoy!