Pulse together the flour, cornflour, salt, icing sugar and butter in a food processor until it comes comes together to form a dough in the machine.
Remove the dough, knead gently and form into a ball. Wrap in clingfilm and chill for 60 minutes. Pre-heat the oven to 180°C. Remove from the fridge and roll out on a lightly floured surface to ½cm thickness. Cut out shapes from the dough using a variety of cookie cutter designs
Grease and line 2 baking trays. Arrange on the baking trays spaced apart and bake for 10-14 minutes until they are golden in colour.
Remove from the oven and allow them to cool on the sheets before transferring to a wire rack to cool completely. Prepare the icing as the cookies cool by whisking together the icing sugar with enough boiling water to create a spreadable icing.
Divide the icing into different bowls and add a little of the food colouring of your choice to each one. Blend again until smooth and even in colour. Spread the cookies with the icing, using a warm, wet knife to help spread it neatly. Decorate some of them with Smarties on top.