Preheat the oven to 200C/180C Fan/Gas 6. Line an 8" square baking tin.
In a small bowl, break up the dark chocolate and melt in the microwave in 20 second intervals, stirring in between to make sure it doesn’t burn.
In a large mixing bowl, whisk together butter and both sugars for 3-5 minutes. Add in the eggs, one at a time and mixing in between. Add the vanilla and mix again.
Now, add the chocolate slowly (check it first, to make sure it isn’t so hot that it will cook the eggs) and combine once again.
Gently fold in the flour and malted hot drink powder. Add the maltesers and combine. Pour the batter into the prepared tin and bake for 30-35 minutes. Let cool on a wire rack. Once cooled, slice and serve.