Beat the spread and caster sugar together in an electric mixer until pale and fluffy. Sift the flour and cornflour together into the mixture, add the vanilla and mix until combined. Roll the dough into a ball, wrap in clingfilm and chill for 30 minutes.
Roll the dough out on a lightly floured surface to about 5 mm (¼ inch) thick. Use a 5 cm (2 inch) square or round cutter to cut out 16 squares or rounds, rerolling the trimmings as necessary. Place on 2 baking sheets lined with baking parchment and bake in a preheated oven, 160°C (325°F), Gas Mark 3, for 10–12 minutes until pale golden.
Leave the shortbreads to cool on the sheets for 10 minutes until firm, then transfer to a wire rack to cool completely.
Sandwich the biscuits together with the jam and dust with icing sugar.