Preheat your oven to 180°C (160°C for fan assisted ovens) / Gas Mark 4. Grease the tin.
Mix the egg, oil, water and brownie mix together until well blended (mixture may be lumpy).
Spread the mixture evenly into the lightly-greased rectangular tin.
Bake in the centre of the oven for 20- 25 minutes or until a rounded knife, inserted 5cm (2”) from the edge of the tin comes out almost clean. After just a couple of minutes, cut circles from the brownie using the top of a glass. Lift the circles out before they stick to the baking tray!
Spread a thin, even layer of icing over the top of one cake and gently press the other cake on top. Cut off a section from each side and a point at the bottom of the cake.
Decorate the caterpillar’s back with colourful sprinkles.
Roll out the orange sugar ready to roll icing to a 4cm width and cut out the face with a glass.
Roll out the white icing a 2cm width, the pink icing to 1cm width and the black icing to 5mm width. Add the 2 balls of white icing to the face and attach the 2 black circles to create the eyes. Then add the pink circle to create the nose. Glue together with some water. Then use a knife to create a smiley mouth. Add the face onto the end of the caterpillar.
Attach The 2 chocolate matchsticks for antennae. Then add chocolate chips for caterpillar feet!
You can place the caterpillar onto a cake board (which can be decorated using ready to roll icing) and add some flowers for a perfect finishing touch to your birthday cake!