Knead butter in flakes, sugar, cinnamon, egg, salt and flour to a smooth dough. Wrap in cling film and chill for about 30 minutes. Roll out to a rectangle (approx. 30 cm x 36 cm) 3-5 mm thick on a floured work surface. Cut into rectangles (5 x 6 cm). Use a square cookie cutter to cut a square from the centre of each rectangle. Place rectangles on 2 baking trays lined with baking paper. Bake one after the other in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for about 10 minutes.
Take out and let cool down. Mix icing sugar, 2-3 tablespoons of water and a few dashes of blue and red food colouring to a purple icing. Carefully spread the icing over the cookies. Finely chop the cranberries and decorate the cookies with them. Let the icing dry. Sprinkle cranberries with sugar if necessary