Preheat oven to 170°C fan (180°C conventional) and line a 20cm square tin with baking paper.
Brown the butter until golden and nutty, then immediately pour it over the chopped chocolates.
Leave for 2 minutes before stirring until smooth. Whisk in both sugars.
Add the eggs one at a time, followed by the vanilla. Fold in the flour, cocoa, and Milo until just combined.
Fold through the chopped Maltesers and pour into prepared tin and bake for 20-30 minutes, until the edges are set but the centre still has a slight wobble.
Allow to cool completely. Melt 75 grams milk chocolate and drizzle all over the top and cover with the chopped maltesers.