Spread the buttercream over the cakes. For each hat, take a small piece of red icing and roll into a log with a point at one end. Flatten using icing sugar-dusted fingers, and position on each cake. Place a Smartie, slightly off centre, for noses. Use red icing to shape curved red mouths and press in position. Roll little balls of green icing and place at the ends of the hats, then roll out small triangles for the beards.