1.Preheat oven to 180°C/350°F. Grease two oven trays; line with baking paper.
2.Beat butter and sifted icing sugar in a medium bowl with an electric mixer until pale and creamy. Add egg and vanilla; beat until well combined. Stir in sifted flours; mix to a soft dough. Gently knead dough on a lightly floured surface until smooth. Shape into a disc, wrap in plastic wrap; refrigerate 30 minutes.
3.Draw a 7cm x 10cm (2¾-inch x 4-inch) rectangle onto the cardboard; cut out template. Roll dough between two sheets of baking paper until 5mm (¼-inch) thick. Using template, cut out rectangles, re-rolling dough scraps until all dough is used. Place shapes on oven trays; bake 15 minutes or until browned lightly. Stand for 10 minutes before transferring to a wire rack to cool.
4.Knead ready-made icing on a surface dusted with a little cornflour until icing loses its stickiness. Roll icing on a cornflour dusted surface until 3mm (1/8-inch) thick. Using same template, cut out icing rectangles; brush backs of the icing rectangles with a little water; stick onto each biscuit. Indent icings on half the biscuits to replicate the back of an envelope.
5.Tint leftover icing red; using the heart-shaped cutter (or a small sharp knife) cut out 2.5cm (1-inch) hearts; brush back of hearts with a tiny drop of water; position on envelopes to represent seals and stamps. Dip a watermark stamp into red food colouring and mark envelope; stand until dry.
Decorate these cookies in colours to suit the event. For weddings and engagements, write the guests’ names on them, and use as place cards or as take-home gifts. You need a 2.5cm (1-inch) heart-shaped cutter, a piece of firm cardboard 12cm (4¾ inches) square, and a watermark stamp.