Preheat the oven to gas mark 4 or 180 degrees and line two trays with baking paper.
Beat together the butter and icing sugar until light and fluffy, then add the vanilla extract.
Gradually add the cornflour and plain flour and mix.
Fill a piping bag (with a star-shaped nozzle) with the mixture.
Pipe a swirl, starting from the centre going outwards onto the tray, similar to icing a cake. The biscuits should be 5cm in size with a few cm between each biscuit.
Bake for approximately 15 minutes until they are pale golden brown in colour.
Leave the biscuits to cool.
Make the buttercream by mixing together the icing sugar and butter.
Pair up the biscuits with those of a similar size/shape. On each of the bases, spread a little jam (approximately one teaspoon), then on the other, a little buttercream, then sandwich them together.
Dust with a little icing sugar to serve. These should last for a couple of days if stored in an airtight container.