Cream the butter and sugar together well and then beat in the egg and essence. Add the flour and Bournvita, and bind the biscuit dough together. Knead on a lightly floured surface until smooth. Roll out, not too thinly and cut out the biscuits, re-rolling the scraps in between. Place on the lightly floured baking tray and bake at Gas Mark 5 / 190?C / 375F for 12-15 minutes, until lightly browned and cooked. Leave to cool before removing from the tray. Melt the chocolate with the oil to make chocolate icing. Carefully dip one end of each biscuit into the chocolate icing and then fill the piping bag with the remainder. Sprinkle on crumbs of Flake for hair. Rub the Buttons a little so they shine and then stick them on with the icing as cauliflower ears. Pipe a variety of faces as illustrated, using simple lines. Leave to set. Triangular coloured paper hats may be popped over the hair, with the names written on them for each guest. NOTES : These cheeky-faced biscuits go down well with small boys, who like to choose the naughtiest! Makes about 20.