Step 1
Use an electric mixer to beat butter, sugar and eggs in a large bowl until just combined. Add the flour and cocoa powder. Beat until mixture just comes together. Turn dough onto a lightly floured surface and gently knead until smooth. Divide into 2 portions and shape each portion into a disc. Cover and place in fridge for 30 mins to rest.
Step 2
Preheat oven to 180°C. Line 2 large baking trays with baking paper. Roll out dough on a lightly floured surface until 3mm thick. Use a small sharp knife to cut out shapes to make a skeleton (draw skeleton shapes, including a skull, eyes, arms, legs and a body, on baking paper and cut out to use as a guide). Place on the lined trays and bake for 10-12 mins or until just firm. Cool on the trays.
Step 3
Meanwhile, to make the royal icing, whisk the egg white and lemon juice in a large bowl. Gradually add the icing sugar, stirring after each addition, until smooth, adding more icing sugar if necessary to make a thick paste.
Step 4
Pipe or spread royal icing over the biscuits. Set aside to set.
Step 5
Use food colouring to tint a little white icing blue, kneading on a surface lightly dusted with cornflour until well combined. Roll out the blue icing to a 3mm-thick disc. Use a small sharp knife to cut out 2 medium discs. Knead a little black icing on a lightly greased surface until smooth. Roll out to a 3mm-thick disc. Cut out 2 small discs.
Step 6
Arrange biscuits on a serving platter to make a skeleton. Arrange blue and black icing discs over the eye biscuits. Use black writing icing to make a mouth. Decorate with red food colouring.