In the bowl of a stand mixer, beat the butter and sugar together until pale and fluffy. Add the egg gradually and continue to beat until combined. Sift in the flour, then add the vanilla extract and mix briefly until the dough is just coming together.
Turn the dough out onto a lightly floured surface and knead gently until completely smooth. Form into a ball, wrap in clingfilm and chill in the fridge for 30 minutes.
Preheat the oven to 180°C / fan 160°C / gas 4. On a piece of baking parchment, draw an egg shape measuring 30cm in length and 20cm at its widest point. Cut out to make a template. Tear off another slightly larger piece of baking parchment.
Remove the dough from the fridge and place it in the middle of the larger piece of baking parchment. Roll out until it's about 4mm thick. Place the template on the dough and cut around it with a knife. Collect the scraps and squeeze back together (this dough can be wrapped in clingfilm and kept in the fridge for a few days if you want to make more cookies at a later date).
Gently take the egg-shaped dough with the baking parchment underneath and lift onto a large baking tray. Bake for 15-18 minutes, or until golden. Remove from the oven and leave the giant biscuit on the tray to cool slightly before transferring to a large wire rack to cool completely.
Now it's time to have fun! Decorate the biscuit however you like using the tubes of icing to draw lines and shapes and as glue to stick on the other decorations – make sure you push them down firmly so they won't fall off. Leave to dry for at least 2 or 3 hours.