Put the butter, icing sugar plain flour, cornflour, and vanilla extract in a food processor and blend until smooth. Scrape the mixture down with a spatula a couple of times
Spoon into a piping bag fitted with a large star nozzle. Pipe 6cm rosettes of dough, spacing well apart.
Bake in the centre of the oven for 13-15 minutes or until pale golden-brown and firm. Cool on the baking tray for five minutes then transfer to a cooling rack.
Leave to cool.
Cream together the butter and icing sugar. Add the vanilla extract and cream if using. Beat until light and smooth. Put in a piping bag with a star nozzle.
Pair the biscuits and turn upside down. Pipe a swirl of icing onto one half of each pair. Put jam on the other biscuit then sandwich together. Makes about 20 biscuits in total. Dust with icing sugar.