Preheat the oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Grease a 20cm square shallow cake tin and line the base and 2 sides with baking paper.
Make, bake and cool the brownies as directed on the box using the oil, water and egg.
Turn the brownie slab out onto a board. Stamp out 9 circles using a 6cm round cookie cutter.
To decorate, put the icing in a medium bowl and use the red food colouring paste or gel to colour the icing bright red. Spread the icing over each brownie circle.
Cut the Catherine wheel into 9 x 6cm strips and 9 x 3cm strips. Using scissors, split one end of each of the long strips by about 2cm. Place 1 down the centre of each brownie, fanning the split end to resemble wings. Use the black writing icing to pipe spots on each wing area.
Attach a biscuit to each brownie (at the base of the wings) for the face and use black writing icing to stick 2 candy eyeballs to each biscuit. Pipe a mouth in white writing icing on each biscuit. Cut each of the short liquorice strips in half lengthways. Create two small holes in the brownie just above the head. Insert a liquorice strip into each hole to resemble feelers; secure with black writing icing. Serve immediately and store any leftover brownies in an airtight container in a cool place for 2-3 days.