Heat the oven to 180C/160C fan/gas 4 and line a 24 x 20cm brownie tin with baking parchment. Melt the butter and dark chocolate in a heatproof bowl set over a small pan of just simmering water. Stir until completely smooth, then leave to cool for 10 mins.
Beat the eggs and sugar with an electric whisk until thick and pale – the mixture should double in volume and leave a trail when the beaters are lifted. Pour the cooled dark chocolate mixture around the edge of the bowl. Sift over the flour and cocoa, add the milk chocolate, and gently fold everything together with a spatula or large metal spoon. Pour into the prepared tin and bake for 35-40 mins.
Meanwhile, make the ‘eyeballs’. Melt the white chocolate in a heatproof bowl over the pan of just simmering water. Stir until smooth. Dip each cookie into the chocolate to cover one side. Stick a sugar-coated chocolate in the centre of each, then leave to set. When the chocolate is fully dry, pipe a pupil and red veins onto each eyeball using the icing pens.
Remove the traybake from the oven, and immediately push the eyeballs into the surface in even rows – do this gently, so they’re just pushed in slightly. Leave to cool completely. Cut into 12 squares.