Preheat your oven to 180°C (160°C Fan, gas mark 4) and line a 9″ squarebaking tray with parchment paper.
Melt butter and chocolate together in a microwavable-safe bowl and leave aside to cool down.
Place caster sugar, eggs and vanilla into a large mixing bowl. With an electric whisk, whisk the mixture until it becomes pale, frothy and has doubled in size.
Pour the dark chocolate and melted butter mixture over the whisked eggs mixture. With a spatula, fold together until combined.
Sift plain flour and cocoa powder over the mixture, then fold again with your spatula until evenly combined.
Pour a pack of Maltesers into the brownie mixture and mix together until evenly distributed.
Pour the brownie mixture into a lined baking tray and spread evenly using a spatula.
Bake in the oven for 25 minutes. Leave the mixture to cool down completely before cutting into it.