Pre-heat the oven to 170°C. Grease and line baking trays with nonstick baking paper. In a mixing bowl, cream the margarine and sugar until pale and fluffy using a handheld electric whisk. Beat in the flour, bicarbonate of soda and vanilla extract and knead this dough lightly for 5 minutes. Roll the dough out on a lightly floured surface to 1cm thickness and cut shapes using a 5-7cm diameter circular cookie cutter. Arrange on the baking trays spaced apart. Bake for 15-20 minutes until golden brown all over. Remove from the oven and let them cool on the baking tray for a few minutes before moving to a wire rack to cool completely. When they are cool, pipe a little icing where the eyes will go; use Smarties for the eyes and press them gently into the icing. Pipe icing where the mouth and ears will go on some of the cookies. Cut and shape some of the banana sweets to resemble mouths and ears. Use the same technique to secure them as you did with the Smarties. Use icing to create mouths and noses for some of the cookies. Finish the cookies by piping the hair at the top of the cookies in different styles. Leave the icing to set before serving them.