Preheat the oven to 190°C / fan 170°C / gas 5. Lightly grease and greaseproof paper line a 23cm/9in square cake tin.
Melt the butter and dark chocolate together in a small saucepan over a gentle heat, stirring occasionally until smooth. Remove from the heat and allow to cool. In a bowl whisk together the eggs and caster sugar, using an electric whisk. Whisk constantly until the mixture has doubled in volume. Fold in the flour and cocoa powder until well incorporated. Then fold in the melted chocolate mixture until well combined. Finally, add the Maltesers Teasers chunks and pour the mixture into the prepared cake tin, spreading it out evenly. Bake for 25-30 minutes, until the top is firm.
Remove from the heat and allow to cool completely. Cut into squares and serve dusted with cocoa powder or icing sugar.