1.Preheat oven to 180°C/350°F. Grease two 8cm x 26cm (3¼-inch x 10½-inch) bar cake pans; line bases and long sides with baking paper, extending paper 5cm (2 inches) above sides.
2.Make cake according to directions on packet. Drop ⅔ cup of mixture into each pan; bake about 20 minutes. Stand cakes in pans 5 minutes before turning, top-side up, onto wire rack to cool.
3.Meanwhile, to make butter cream, beat the butter (and any flavouring, if using) in a small narrow bowl with an electric mixer until the butter is as white as possible (this will result in a whiter butter cream which will give you better results if colouring it). Gradually beat in half the sifted icing sugar then the milk, then the remaining icing sugar. Beat until the butter cream is smooth and spreadable. Keep scraping down the side of the bowl during beating.
4.Cut strips of licorice strap into 16 x 3cm (1¼-inch) pieces. Tint butter cream white. Spread butter cream over tops of cakes; cut each cake crossways into eight 3cm-wide pieces. Secure cakes on board with a little butter cream.
5.Using picture as a guide, position licorice strips on cake pieces; decorate with mini M&Ms before the butter cream sets.
EQUIPMENT: two 8cm x 26cm (3¼-inch x 10½-inch) bar cake pans; 35cm x 50cm (14-inch x 20-inch) rectangular cake board Use only ⅔ cup of the cake mixture in each cake pan; the cakes must be shallow and flat on top. You will only have a tiny amount of mixture left.