Line a baking tray with baking paper and set aside.
Roughly chop the Maltesers into a mixture of small and large pieces.
Melt the butter and set aside for 10 minutes to cool.
Add the sugar, brown sugar, flour, cornflour, cocoa, baking soda, malted milk powder, salt, chocolate chips and chopped Maltesers into a bowl and mix together.
Pour in the melted butter, egg and vanilla extract and mix into a dough.
Roll into 12 - 16 balls and place on the baking sheet.
Refrigerate for an hour.
Preheat the oven to 180°C (350°F) and place the cookies in the oven for 12-15 minutes.
The outsides will be browning and the middle will be soft but it will harden as they cool.